On the morning of 15 March, Joy left Croydon’s Gilroy Court Hotel, and then she disappeared. No one knows what happened next: whom she met, where she was taken, whether she even went left or right. She had no family, no apparent friends, no one whom the 17-year-old could trust.

Little is known about Joy. All we have is a handful of biographical snippets from her brief interviews with police and social services. Conducted in broken English – her first language is Edo – they explain that she was born on 13 July 1993 into the poverty of rural Nigeria. It was a far-from-innocent childhood. “Her parents attempted to sell her to an 85-year-old man whom she did not know,” one transcript read.

Despite the austere language of the documents – a summation of police case notes leaked to the Observer – they portray a resourceful, determined but acutely vulnerable figure. They chronicle Joy’s escape to another Nigerian town where she survived by selling newspapers on the street. There, she met the man who would change her life. From the documents it is evident the character, referred to only as Steve, is a human trafficker.

He promised Joy a new life in Europe. “She agreed and got on a plane with him,” reads a police entry. Aged 14, she entered the UK, but the UK Borders Agency has no documentation of her arrival. For the first three years of Joy’s life in England there is absolutely no record of her existence.

According to the interviews, her fresh start soon soured. Upon arriving in the UK, Steve confiscated her passport, an established trafficking tactic to augment control over victims.

What happened next is covered in the briefest of detail, but Joy managed to “lose” Steve during the following years, finding work as a church cleaner before sliding into Britain’s vast but secretive underground sex industry to “feed herself and survive”.

Joy claimed to have merely responded to an advert for work at a Croydon massage parlour at the start of 2011, but the sordid surroundings where she was found, aged 17, in the days before the interviews were conducted – with its Fort Knox obsession with security, its reinforced doors and barred windows – suggest she was a captive sex slave.

In the month Joy went missing there were 491 unaccompanied children seeking asylum in Croydon and it is these, say police, who are most vulnerable to traffickers. No one knows how many vanish without trace into the sex trade, but local campaigners estimate that up to two trafficking victims can be found for each of the borough’s 40 or so brothels. In London, a further 2,103 illegal massage parlours and sex shops have been identified by police intelligence. Few of these victims ever escape. Had it not been for a phonecall on Saturday 6 March this year it is almost certain Joy would never have been noticed.

Clearly anxious, the female caller described a teenage girl detained “against her will at an address in Croydon”.

The officers who arrived saw that the three-bedroom house was no regular property. Steel bars shielded its windows, an iron gate guarded a reinforced front door. Inside, a generous network of CCTV cameras monitored its corridors. The solid-steel entrance “looked like a police cell door”. Whoever owned the place was either paranoid of intruders or keen to keep people from escaping.

In one of the back rooms they found two people: a 5ft, slightly built teenager from Africa with a “punter” – a local man seeking paid-for sex. It was evident, say police, that the teenager’s welfare was at risk. They were relieved. They had liberated a child from the sex trade.

Soon she would be safe, or at least that was the idea. Joy was placed in the care of Croydon Children’s Services and a bed found in the Gilroy Court hotel. This is a “hotel” which, behind its cream-coloured exterior, has numerous crowded rooms where the state sends its asylum seekers, refugees and those who most require the safety net of the state.

An investigation by local campaigners recently identified 12 brothels within a 10-minute walk of the Gilroy Court, and others are almost certainly still to be identified. The concentration is, according to local charity Croydon Community Against Trafficking, partly explained by the proximity of Lunar House, headquarters of the UK Borders Agency. More than 9,000 asylum seekers visit the building each year, many placed in establishments like the Gilroy Court while their applications are assessed.

An unquantifiable number are lured into the sex trade. Of six Croydon brothels contacted one night last month, the women on duty represent a league of nations: three Brazilian, two Spanish, two Puerto Rican, two Chinese, one Thai, one Turkish, an Italian, a Pole and a Colombian. Establishments typically operate under three different names with as many telephone numbers, but the address remains the same, and so, too, do the women.

“Their busiest times are the mornings when men travel to work and the early evenings when they return home; there’s a large but secret commuter trade,” says Michael Darby, a volunteer with Croydon’s anti-trafficking community group. But Joy, according to the information she gave police, was available 24/7, averaging two customers a day for sex. At night she received “protection” from a man she knew as Matey. Clients paid £60, of which Joy was allowed to keep £20 with the brothel taking £30 and a maid – her pimp – another tenner.

If she told the truth to the investigators, Joy appears to have been treated relatively well compared to others trapped in Britain’s frequently violent sex trade. Scotland Yard estimates that trafficked victims, on average, are forced to sleep with seven clients a day. In addition, Joy was allowed to use condoms, keep a portion of her earnings and never admitted to being beaten or tortured.

Joy’s ordeal was meant to have ended following her rescue. But the supposed sanctuary offered by the Gilroy Court Hotel, advertised online as a three-star establishment, would prove false.

When the Observer visited the hotel last month the small reception area was chaotic: pockets of men milled about the entrance, a continual procession of people trooped in and out. A security guard explained that his job was to protect residents not just from strangers outside but from those within. “Criminals come in here, people straight from prison, not good,” he said. Gesturing towards an internal corridor, he added: “We have about 300 in there at the moment.”

Officers from Croydon’s missing persons unit admit they would not want a child they knew being sent to the Gilroy Court. “They have got so many people coming and going, a lot of overnighters,” explained one. “It’s not a place I’d want anyone being put in.”

There’s no evidence connecting the hotel with traffickers, but child protection experts say it is naive to assume that Joy would not have been targeted while at the Gilroy Court.

Word would have spread quickly that a teenager linked to the sex trade was on her own. From a criminal perspective, Joy was a valuable commodity: she was worth £21,450 a year to whoever controlled her at the Croydon brothel where she was found. “There is no way she should have been anywhere near that place,” says Andy Elvin, chief executive of the charity Children and Families Across Borders, shaking his head as the circumstances of Joy’s predicament are discussed.

Parliament heard last year that some state accommodation was so unsafe they effectively doubled as “holding pens” for traffickers preying upon the most vulnerable residents: children. A Croydon council spokesman cautioned against naming the Gilroy Court, warning “predatory people” were more likely to “hang out nearby” if it was known vulnerable people were inside.

Two days after Joy was rescued from her suburban brothel, police called the Gilroy Court Hotel saying they wanted to interview her. Officers were keen to shed light on her alleged traffickers, but they were too late. She had already gone.

That might have been that but for an early break in the resultant police investigation: a local pimp revealed text contact with Joy. On 11 March the teenager was tracked down to a brothel in affluent Surbiton, Surrey. She was found cowering under a bed.

This time officers arrested Joy on suspicion of immigration offences. Her fingerprints were taken, along with a police photograph, the only known picture of Joy. Wide-eyed and clearly apprehensive, she stares back at the camera, petrified. Maybe she sensed what was about to happen.

At this stage police also appeared increasingly concerned for her safety. When officers from the local child exploitation unit contacted Croydon Children’s Services with the news they’d again plucked a child from the sex trade, attempts were made to ensure she was safe. This time, according to the documents, they handed over Joy on one crucial condition. Officers issued a “request that she was not placed back at the Gilroy Court Hotel”.

The demand went unheeded. During her second stay at the hotel there’s evidence Joy was distressed. Two days after her arrival the teenager called Croydon police asking for a favour.

“She made direct contact with the missing person unit at Croydon and requested she be collected and brought to the police station,” according to the documents.

Something was clearly amiss. Had she been tracked down by her pimps? Officers responded by asking social services to visit her, but according to the paperwork no official was available. Instead, police went to the Gilroy Court and explained to Joy that a Home Office meeting was arranged for three days later. “She appeared to fully understand… [she] requested we collect her and take her to the interview which was agreed,” a statement reads.

Later that day, a social worker describes Joy as happy with her situation. “She felt safe in the hotel and in the surrounding area of Croydon.” The following day, 15 March, Joy left the hotel for a meeting with a social worker, walked down to the London Road, and, as the documents noted, “has not been seen since”.

A mile east of the Gilroy Court lies the Ashburton playing fields where dog-walkers and joggers congregate each morning. On 26 July 2008, a hot summer’s day, Lena Monteau went to play there. The 14-year-old Romanian had no money and could barely speak English. She has not been seen since. At the time, detectives warned of people in the area who “take advantage” of foreign children. “We deal with quite a high number of cases where young people in this situation are sexually exploited,” states one police briefing shortly after Lena went missing.

Monteau’s case is among those fuelling a growing consensus among child-protection specialists that not enough effort is being made to find missing foreign children. “They are reported missing, but no one really looks, no one really cares,” says Elvin.

Inside the reception of the Gilroy Court Hotel last month there were no “missing persons” posters of Joy, no indication she had ever been there. Among the staff questioned, none recognised her picture let alone knew her name. “We’ve had people who have stayed in there for weeks and they never know who they are,” said a police source, a claim denied by the hotel.

A poster of Joy was handed to staff in the hope it might spark a lead. When contacted four weeks later, the hotel manager offered: “We don’t know where she went or what happened. She went to see her social worker and never came back.”

If Joy was white and British, says Elvin, such a response would be improbable. Too many investigations, he believes, are wound down before they even get started. One issue is that police forces have no “clear-up targets” for locating missing trafficked children. Normally there are no family and friends campaigning for information. Even with a committed police team actively seeking leads, Joy’s case demonstrates the challenges of finding a child who might be held by criminals.

Both the UK Human Trafficking Centre and SCD9, Scotland Yard’s anti-trafficking unit, are aware of the Nigerian’s disappearance but, intriguingly, no mention is made of the involvement of the Child Exploitation and Online Protection Centre (CEOP).

On 1 July CEOP was handed responsibility for leading attempts to find missing children, promising “rapid and effective conclusions” to complex or high-profile cases, but only one specific appeal for a missing child currently appears on the agency’s website: that of Madeleine McCann. Instead, the official missing kids website says the search for Joy is being led by the National Police Improvement Agency, a body about to be axed by the government.

It is this casual approach, says Elvin, that demonstrates how foreign child-trafficking victims receive “a second-rate service” compared to British cases.

Scotland Yard’s media bureau, which frequently circulates appeals for missing people, has yet to issue a request for information regarding Joy, seven months after her disappearance. By contrast, a media-wide appeal was launched when a British woman from Croydon went missing in September. She was found the next day.

Jim Gamble is an expert in using intelligence to track down children. A former head of CEOP, he led the National Criminal Intelligence Service’s fight against child sex abuse and, before that, Northern Ireland’s anti-terrorist intelligence unit in Belfast. He agrees that attempts to locate foreign child-trafficking victims are cursory, at best. “The true extent of the government’s commitment at the minute is simply making sure a picture goes up that no one ever sees. How do we reach these children? If we’re going to send messages then, for example, we should send them in a foreign language.”

Several weeks before Joy disappeared, Croydon police received another tip-off. Intelligence indicated that a residential home had been converted into a drugs den. Inside the Upper Norwood property – a 15-minute walk from the Gilroy Court – officers stumbled upon something unexpected. Hidden among 330 cannabis plants was a small Vietnamese child.

She was called Vananh Nguyen and, at just 11 years old, could barely speak English. Croydon Children’s Services placed her into a foster home.

Police agreed it was highly probable that Vananh was trafficked into London by organised criminals. On 2 February a meeting between detectives and the child was scheduled to help glean information on the gang that had forced her to tend their cannabis crop. The day before, Vananh vanished – she has not been seen since.

It’s plausible the traffickers may have reached her first. Vietnamese trafficking gangs are notoriously adept at recapturing victims from care. Philip Ishola of the London Safeguarding Children’s Board says that from his experience 90% of Vietnamese children in care go missing. “The traffickers exploit fear effectively,” he explains. “One boy had his face pressed down on a hob to ensure he never called the police.”

Once they are gone, few resurface. Child protection experts say that placing trafficked children, like Joy or Vananh, in the same locale as their captors exacerbates the risk of being caught. But Croydon council knew that. In the month Vananh vanished, the London Safeguarding Children’s Board issued guidance to all borough councils warning of the risk of suspected trafficked children disappearing. Many go missing, it warned, within 24 hours of arriving in care.

Gamble identifies an initial “48-hour window” when a child is effectively saved or not. Joy survived slightly longer, lasting just over 70 hours in the Gilroy Court before disappearing. Many disappear in broad daylight.

The chances of finding Vananh appear even more remote than finding Joy. The 11-year-old does not warrant a mention on the missing people or police websites. No photograph of her exists, her description is not available. It took six weeks after she went missing before a report appeared in the local paper, coincidentally the day before Joy vanished.

The National Referral Mechanism (NRM), introduced in 2009 to protect trafficked victims by rapidly identifying possible cases to the UK Border Agency, was supposed to change a system which a growing number of child-protection specialists believes discriminates against foreign children in favour of British youngsters. Joy was referred to the NRM, but prompt protection was neither apparent or forthcoming.

One issue, say critics, is that the NRM is staffed entirely by Home Office officials; not a single trafficking or child protection specialist has input. Instead of concentrating on a child’s welfare, victims are often arrested. Vananh initially faced drugs charges, while Joy was accused of immigration offences, a decision that may have inadvertently provoked such distrust towards the authorities it helped drive her back into the underground sex trade.

Elvin believes the system has become so weighted against foreign youngsters that an “institutional racism” is palpable, an allegation partially corroborated by official data. Freedom of Information responses show that NRM officials believe three-quarters of suspected trafficking cases involve British citizens, compared to a third of EU cases and 11.9% of those from outside Europe. Children are equally unlikely to be believed. Just 32 foreign children out of 187 referred were granted a “positive decision”, meaning they are placed within a government safe house.

Elements of Joy’s account appear to have been contested by Croydon council, particularly her age. “There were suspicions she was older than she claimed to be,” said a spokesman, “and she did not co-operate with investigations while she was staying with us.”

Some local authorities even admit to discriminating against foreign children. Solihull council has unveiled plans for teenage asylum seekers to be fast-tracked from foster care, but not British children. Council officials concede the creation of a “two-tier service” that may mean its legal duty to foreign youngsters “will not be fully met”.

Elvin cites a recent case where a 13-year-old Vietnamese girl, pregnant after being raped in a brothel, was placed in a hostel of adult men. It is unthinkable, he said, that a suspected trafficked child from the UK would end up in the Gilroy Court Hotel.

It is almost impossible to come by facts – there is no centrally held data on actual numbers of youngsters trafficked into the UK and ministers refuse to launch a review into the number of unaccompanied youngsters missing from care. A rare glimpse into the potential scale are the statistics obtained through a Freedom of Information request that show 330 children aged between nine and 17 disappeared during the 14 months up to summer 2009, more than four a week. How many are recovered is anybody’s guess.

Requests for information from councils are routinely ignored. Portsmouth, Southampton, Kent and Manchester were asked how many overseas children went missing from their care over the past year. Only the last replied, claiming three. Kent, known to have lost 173 unaccompanied asylum-seeking children from its care two years ago, disregarded repeated requests despite being given a list of those missing from its area. Croydon council says that in the past year 11 unaccompanied asylum-seeking children disappeared from care, a figure that does not include Joy because she never sought asylum. “There is no push for finding these children,” says Elvin. “Tens of thousands could be here without the knowledge of the local authority.”

Late last month, the Observer returned to Croydon looking for clues to Joy’s whereabouts. Even among the brothels, no one claimed to know her. A pimp at Estrella’s, behind Richer Sounds on Ledbury Place, said: “We don’t employ blacks.” Another, around the corner, said they only employed Europeans. But then, through an archway off Penge Road, came a breakthrough.

The maid at Honey Pots Massage, a bungalow two miles north of where the youngster was last seen, confirmed Joy had worked there – but we had arrived too late.

“She’s left.” Where to? “Don’t know, but she’s gone.” Without missing a beat, she continued, “But we have two other lovely black girls here. Lisa who is Caribbean, long hair and 5ft. Slim, double D, size 8 to 10 dress and then we have Ella, a pretty girl with long hair, curvy bust, double D, size 12.”

The lead soon went cold. Nearby shopkeepers shrugged at her picture, pimps shook their heads. The capital’s Nigerian association shared no clues. The official missing-people website advised following updates on her case, but no updates materialised. “We’ll be in touch when the appeal for Joy closes to ask you to take the poster down,” read a statement.

Croydon council, which insists Joy was “dealt with by the book” and rejects claims the hotel was unsuitable, does not believe she was abducted. A spokesman said she “appears to have most likely moved on of her own volition”.

No blame should be attached to the Gilroy Court Hotel, say experts: its role was simply to receive bookings from probation, social services and other state agencies in a place where people were free to come and go. The manager of the Gilroy Court said the hotel had been running for 30 years without incident. “Nobody has ever gone missing,” she said. “Nothing has happened.”

The police, though, are increasingly frantic, their search complicated by several aliases Joy uses or has been given. “We found her twice,” argues a source, “and maybe she hasn’t been dealt with in the way that she should have been. But she wasn’t around long enough for anyone to do that. We need to track her down.”

ON Wednesday David Cameron will host a champagne reception at Downing Street to mark anti-slavery day. Among the guests will be immigration minister Damian Green who, unveiling the government’s anti-trafficking strategy six days after Joy’s 18th birthday, promised to “ensure that children remain a focus of our efforts as we look to combat these traffickers who exploit vulnerable children”. During the three months since the minister’s statement, there has not been a single UK conviction for child trafficking. The organised criminals, traffickers and brothel owners in the case of Joy, Vananh or Lena are also likely to evade prosecution.

Away from the clinking of glasses at Number 10, the search for Joy will continue. But the challenge is harder than ever. Now 18, Joy is finally legally able to decide her fate even if in reality her destiny is beyond her control. The initial police inquiry into a missing “looked-after child” has been superseded by an investigation to locate the whereabouts of a young trafficking victim.

It is impossible to know what will become of Joy. She may have been re-trafficked out of the country on a false passport. She may be in a different city. She may be dead. She has no family, no guardian angel. All she ever had was Britain’s obligation to help a vulnerable child. And that was never enough.

• The following correction was published on 23 October 2011:

“Joy vanished into Britain’s child-sex trade” (Magazine) referred to Solihull council fast-tracking teenage asylum seekers out of foster care, “but not British children”. We should clarify that the young people go from foster care into supported accommodation. Those who are still at school or have needs that can only be met in foster care will remain there. This also applies to local children.