Odd place, Britain. Every day, 13 million CCTV cameras track our movements. We're PIN-numbered, databased, credit-rated, nannied, Neighbourhood Watched, Facebooked, emailed and GPS-ed. You wouldn't think any of us could slip away unnoticed. But we do, in ever-increasing quantities. An Independent on Sunday investigation has established that the numbers of Britons who disappear each year is now at record levels.

Missing People, the charity that helps both the disappeared and those left behind, told us that 250,000 missing persons reports each year – more than 30,000 higher than any previous total – is "probably an underestimate"; others put the total nearer 275,000. This, the equivalent of the entire population of Plymouth being spirited away, means that, across the country, one person goes missing every two minutes. The vast majority are swiftly found, or return of their own volition, but many don't. Some disappear for decades, and sources, including some inside the police, say the number of people in Britain who have been missing from family, friends and usual haunts for more than a year is at least 16,000 and could be as many as 20,000.

Among them are people like Melanie Hall, last seen in a Bathclub nightclub in 1996, whose parents had to endure 13 years of waiting and wondering before her remains were found, a week ago, beside the M5. She had been murdered. Nor does death always bring closure. At any one time, there are an estimated 1,000 unidentified bodies lying in the country's mortuaries and hospitals. Many have been there for years – unknown, unclaimed citizens.

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The long-term missing inhabit a looking-over-their-shoulder world of false names, cash-in-hand jobs, hostels and short lets. For their families, they leave behind not only trauma, grief, guilt, anger and despair, but also, if they are breadwinners, more practical problems. Missing people are deemed neither dead nor properly alive, so salaries are stopped, insurance companies won't pay out, bills can't be paid and corporate "helplines" won't discuss the disappeared's affairs because of the Data Protection Act. But, most of all, the long-term missing leave behind an aching sense of mystery: what has become of them, and why did they go?

This is the story of Britain's long-term disappeared – of people such as Joyce Wells, Alan Hobbs and Janet Cowley; of those as young as seven-year-old Daniel Entwhistle, missing from his Great Yarmouth home since May 2003, or as elderly as 88-year-old Mary Ferns, missing from West Lothian for 16 months now. All an agonising riddle. Why did the Gloucester librarian Angela Bradley leave her spectacles in her car, the keys in the ignition, and walk away one January day in 1995? What happened last November to Quentin Adams, a 40-year-old father of three from Banchory, Aberdeenshire? He popped out to buy cigarettes and has not been seen since. And where on earth is the 14-year-old Doncaster schoolboy Andrew Gosden?

Some 93 per cent of the children who go missing do not live in a two-parent household, and single children are more likely to run away than those with brothers and sisters. Andrew fell into neither category, happily living, according to testimony from his caring family, with his mother, father and elder sister, Charlie. He was doing well at school, and no one had noticed him behaving in any way that would set alarm bells ringing. And yet, one day two Septembers ago, he left for school, waited for his parents to go to their work as speech therapists, returned to the house, changed his clothes, went to a cash machine, withdrew £200 of his savings, and boarded a train to London. We know this because he was seen on CCTV arriving at King's Cross, a slight figure dressed in black jeans and T-shirt. No one has seen him since. The despair, the not knowing, hit his father, Kevin, like a truck. He tried to commit suicide, hanging himself from the banisters, and his life was saved only because the vicar – who had a key to the house – arrived at that moment.

The efforts to find Andrew could not have been greater. Police were swiftly alerted, as was Missing People and local media. His face is on the web, on posters, and on 15,000 leaflets that were distributed in London by three coachloads of family, friends, schoolmates and teachers, who travelled to London and searched for him a year after his disappearance. His 14-year-old face stares from a page on the Missing People website, increasingly a reminder of what he once was, rather than an aid to recognising him now. The Andrew who left the house in his school uniform is no longer the Andrew who might be found. So an age-progressed face will feature on a new leaflet, to be emailed to snooker halls and, if permission is granted, to be handed out at a Muse gig, one of Andrew's favoured bands.

Back in Doncaster, his family keep his childish things, and the clothes that will no longer fit him, in a room unchanged since that day in September 2007. They can still look and hope. What they cannot do is grieve. Kevin Gosden told us: "We have all reacted differently in our house. It's been a battle with depression for me. I haven't reached the point where I can give up – there's always another chance to find him. Sometimes it feels like we're going round and round in circles, like we're trapped in a work by Escher."

Children make up the bulk of the missing persons reports in Britain. But, as teenagers who stay out a night or two from their care or foster home, or who sleep on a friend's sofa to cool down after a row with a parent, they are also likely to be the cases that are resolved within a few days. Teenage runaways are overwhelmingly female: 71 per cent of missing 13- to 17-year-olds are girls. With adults, it is different. Men predominate, with 73 per cent of all disappeared people over the age of 24 being male. Adult missing cases are also far less likely to be resolved quickly, or at all. A 2003 study found that only 20 per cent of missing adults traced by Missing People decided to return to the place they had left, and 41 per cent of those located were not prepared to make contact with those who were looking for them. The conclusion is that they're fleeing something – in their own minds or in reality – far more deep-seated than the cause of a teenager's tiff with Mum, Dad, a step-parent or friends.

There have always been the elderly and confused, the alcoholics, drug addicts and obsessive loners who drift out of contact, until the family, wishing to try again, finds there is no forwarding address. And there will always be the utterly inexplicable disappearances – people such as

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Anne Simpson, a mother of 60, who went for a walk near her home in Ingoldmells, Lincolnshire, in September 2004 and never returned.

But the most intriguing of the missing are those ordinary folk who have some discernible pressure in their lives, but one which seems on the surface no worse than that experienced by the millions who simply keep battling on. It might be job stress, money worries (the recession is a major cause of a rise in missings), or relationship breakdown. But what is it that tips them over some invisible edge and compels them to make a sudden bolt for the door? And what is it like to be the family left behind?

To find out, we sat down with Anne and Peter Langridge, sister and nephew of Bernard Coomber, who went missing in January this year. His story contains many of the ingredients of other missing cases. You could call it "A Very Average Disappearance".

Bernard was 55, unmarried, and lived alone in Sevenoaks, Kent. He was an outdoor person, who often went walking and the job he liked best was landscape gardening. "That was his first love," says Anne, "but he had back problems, so he went into a factory that made parts for showers. He worked for an agency that made him redundant; he was taken on again when the work picked up, then they made him redundant again." By early this year, he had not worked for two years and "had totally run out of money". So she gave him £50.

One day in late January Anne was called by one of Bernard's neighbours. The woman could get no reply at his house. Anne went round, let herself in, and found the house empty. On the kitchen table were laid out Bernard's mobile, and beside it was the £50 Anne had lent him. He was, she explained, a proud man and hated accepting money.

"He took nothing with him," says Anne. "Not a bank card, small change, not a rucksack or holdall. He just walked out with whatever he'd got on. His coats were still in the house. And it was a bitterly cold day."

It was, in a phrase used by so many families of the missing, "totally out of character". Peter says: "He was a loner, really. He led a simple life, but he was quite a grounded sort of person." He was, however, "a bit down, having problems finding a job", says Peter. And, like many on benefits, things did not run smoothly. Anne says: "He had flu at Christmas and, because he didn't sign on by phone, they signed him off and he didn't get his money. So, within a month, there was no money coming in ... he didn't get on with the man at the Jobcentre and wanted to be referred to another one, but they wouldn't allow that." Bernard's last words to Anne were: "I've got myself in a mess, and I'll get myself out of it."

Like quite a few of the mature missing, Bernard had been a sort of carer, to his father, who died seven years ago. "Bernard did have one girlfriend," says Anne, "but, sadly, my dad made that one fizzle out. He was frightened of being left on his own." Instead, with his money problems, bad back and a troublesome recent hernia operation, it was Bernard who was left on his own.

Kent Police have carried out extensive searches, traced all possible contacts, travelled to interview Bernard's friends up north, talked to his doctor, publicised his details, and checked any bodies that have turned up. Appeals have appeared in local newspapers, on the net, in The Big Issue, and on posters besides the paths where he used to walk. But nothing. Anne says: "My only feeling is that he may have taken his own life in the old quarry, where he knew he wouldn't be found, because he wouldn't want to put me through the cost of a funeral. If he's taken his own life, he'll have put himself somewhere we won't find him for a long time."

As soon as the leaves are off the trees, police will use a helicopter with thermal-imaging equipment to see if any remains can be found in Bernard's favourite rural spots. Anne and Peter say that Missing People (who call regularly), and the police, both the Kent force and the National Policing Improvement Agency's missing persons bureau, "could not have done more".

The offices of the charity Missing People are the closest this country has to a nerve centre for the disappeared. Above a supermarket on a busy west London street is an operation that looks like a police incident room. Phones are constantly manned, and, on the wall, there are wipeboards with lists of names, and when and where they were last seen. Missing People, founded 20 years ago in the wake of the disappearance of Suzy Lamplugh, who lived near by, runs three helplines – for young runaways, missing adults, and the families of the disappeared, all manned 24 hours a day. They receive 120,000 calls a year. The chief executive, Martin Houghton-Brown, says they can barely cope with the volume.

In the early hours of last Monday morning, for instance, the two volunteers had 30 calls in an hour. They included sightings, relatives making initial reports and the missing phoning in. A recent sample: "James", 13, missing from care and sleeping rough on a park bench, angry and upset, who agreed to be put in touch with a social worker; "Paula", a long-term disappeared who had swallowed a large amount of paracetamol and drink, who eventually allowed Missing People to call an ambulance; "Adrian", 50, who had walked out on his wife, but wanted to let her know he was safe; and "Aina", 24, from Bradford, whose parents had her booked on a flight that night to go to a forced marriage. She was frantic; Missing People put her in touch with organisations such as the Asian Women's Domestic Helpline.

Mr Houghton-Brown and his policy and research director, Geoff Newiss, are clear about what needs to be done to help Britain's missing and their families. First, a government department needs to take responsibility for the issue. Second, comprehensive information on the missing needs collating and analysing centrally (we are better at keeping tabs on missing cars than missing people, according to Helen Southworth, Labour MP for Warrington South and a long-time campaigner for the missing). Third, all agencies must have a duty to co-operate. And, fourth, underpinning all this, these responsibilities need to be statutory. "It means resources," says Mr Houghton-Brown, "but we're talking about people dying every day."

Adults, unless illegality is involved, have a perfect right to go missing, assume a new identity, and live out of contact with their former friends and family. (One man who disappeared told Missing People when he was traced: "How dare you look for me!" – and threatened to sue.) This has fed the myth that the police regard any missing case which is not that of a child, or where a crime is suspected, as beyond their remit. It may once have been true, but not now. In Bramshill, Hampshire, the NPIA's missing persons bureau logs and helps investigate cases. And it is thanks, in part, to its work that families such as Bernard Coomber's testify to the lengths to which most forces go to find their lost loved one.

Down in Surrey, police still keep active Operation Scholar, the search for Ruth Wilson, a sixth-former who went missing 14 years ago. She left Dorking just after 4pm on 27 November 1995, and, instead of going home, took a taxi to an isolated pub on Box Hill. Intriguingly, she had ordered flowers for her parents to be delivered two days later. More significantly, police later learned that Ruth, the bookish-looking daughter of two teachers, was in the habit of going to the remote spot on the way home from school. (As an example of the almost limitless trials facing families of missing persons, the Wilsons were asked if they were willing to appear on a game show where the audience would vote on the best step the family could next take to try to get their daughter back. They declined.)

Although Missing People uses a specialist in age-progressed likenesses to portray people missing over the long term, there is a limit to what it, and the police, can do. So families hand out leaflets, put up posters, tramp the streets, offer rewards (£10,000 is not an uncommon amount), hire private investigators (an extensive search can cost more than £15,000), and even, as Kent Police told us, consult mediums. They also start groups on Facebook, and launch websites such as the one for Nicola Payne, who went off to collect clothes for her baby in December 1991, took a short cut across fields, and has not been seen since. Among the poignant messages on the site is one from her son Owen – now 17, but just seven months old when his mother disappeared: "I envy my older cousins who remember her well, and they tell me what a fun-loving girl she was... My one wish would be to have my mum found and to be able to understand the confusion, mystery and heartbreak of the past 17 years."

Some do return. About 10 disappeared persons a week are found through the work of Missing People, among them Billy Andrews, who went missing from his family after his marriage broke up. He began sleeping rough, and defied all the efforts of his mother, Kathleen, and his four sisters to find him. Twelve years went by, and then Kathleen saw an advertisement for Missing People and rang. Within four weeks, the charity's case managers had found him. Kathleen says: "One day I was watching my favourite soap when the phone rang. It was Billy. We both wept." Billy says: "I was so happy when I got the phone call from Missing People telling me that my mum was trying to find me. To be back in touch with her and my sisters after so long was a dream come true." So why did he lose touch? He felt he had let them down and was ashamed of the state he was in. He is now settled, and has remarried. "It is," says Kathleen, "a second chance for all of us."

Thousands of Billies, Bernards, Ruths and Andrews will join the ranks of the long-term missing this year. Maybe it isn't so curious that they can elude all the tabs kept on us, all of our petty nannyings and risk assessments. We may have officials logging missing cars, we might microchip our dogs, and indelibly mark our possessions, but we're awfully casual about lost humans. After all, in 2009 there is no government department responsible for listing and finding them. Odd place, Britain.

The trafficked girls: They all exhibit a vulnerable prettiness

Among the passport pictures of the disappeared staring out from the Missing People web pages a sizeable number are of teenage girls of Far Eastern origin. Xia Wang, 17, has been missing from Westcliff-on-Sea, Essex, since November 2006; Qin Wang, 16, from Bournemouth since January 2007; Yan He, 17, from Worthing since July 2007; Dung Thi Nguyen, 17, from Catford since April 2007; Lihua Hi, 16, from Birmingham since June 2006. There are many others. Having been brought to this country illegally, such girls – whose only common characteristic, says Missing People, is their region of origin and their vulnerable prettiness – are warned by those who transported them to trust absolutely no one. They are taken into care, but, a short while later, are often seen getting into a car driven by an older male oriental. They have been trafficked.

Britain's unclaimed bodies: They lie refrigerated in Britain's mortuaries

Who was the man known as Mr Seagull, whose body was found on Chesil Beach, Dorset, in 2002? Who was the white man aged between 30 and 40 killed at Canterbury by the London-bound train in October 2001? Who was the man whose badly burnt remains were found on Parley Common, Dorset, when firefighters tackled a heathland blaze in August 2006? Their bodies, and hundreds more, lie refrigerated in Britain's mortuaries, awaiting identification. One reason there are so many is because there is no database of the DNA of missing people, which Dr Tim Clayton of the Forensic Science Service has described as "a national disgrace". And an investigation in Scotland by the Daily Record last January found that police there have the DNA of just 34 of 450 long-term missing cases on their books.

THE DISAPPEARED...

Kevin Fasting

Age at disappearance: 50

Last Seen: 21 November 2003, leaving his Merseyside home for work.

Background: The father of three called himself "the worst father in the world" in a note found after he went missing.

Laura Haines

Age at Disappearance: 30

Last Seen: At her home in Bristol on 23 February 1997.

Background: Laura left two daughters behind. Investigators have looked into whether her disappearance is linked to previous relationship break-ups.

Alexander Sloley

Age at Disappearance: 16

Last Seen: Alexander was last seen by a friend in Edmonton, north London, on 2 August 2008.

Background: Alexander's was one of the first cases to be publicised on nearly 13.5m milk cartons at Iceland, the supermarket chain.

Quentin Adams

Age at Disappearance: 40

Last Seen: Buying cigarettes in Banchory on 6 November 2008.

Background: The used-car salesman had been living with his sister, and left three children behind. He disappeared without his mobile phone or passport.

Joyce Wells

Age at disappearance: 72

Last Seen: At her Bexhill home on 22 November 2008.

Background: Joyce was about to visit her daughter but failed to make the trip. She left personal effects, including her handbag, behind.

Luke Durbin

Age at Disappearance: 19

Last Seen: Luke was last seen early on 12 May 2006 after a night clubbing with friends in Ipswich.

Background: Luke had gone missing before, though only for one week and in that time he had remained in contact with his sister. His mother has led the media campaign to locate him, appearing on TV appeals on numerous occasions.

Liz Chau

Age at Disappearance: 19

Last Seen: Walking to her home in West Ealing, London, 16 April 1999.

Background: Liz, a student at Thames Valley University, went missing shortly after handing in coursework and meeting a friend for a drink.

Bernard Coomber

Age at disappearance: 54

Last Seen: Around 10 January 2008, near his home in Kent.

Background: Struggled with unemployment. 'Missing' status means Anne, his sister, cannot sell or let his house.

Robbie Carroll

Age at Disappearance: 40

Last Seen: He disappeared from his home in Lincolnshire on 20 February 2006.

Background: The Cambridge graduate, who specialised in Italian Renaissance literature, had appeared unwell, according to friends. He was badly affected by the death of his mother.

Nicola Payne

Age at Disappearance: 18

Last Seen: Leaving her parents' home in Coventry on 14 December 1991.

Background: A family website carries messages. A man was arrested in 2007, but the case is still open.

James Nutley

Age at Disappearance: 25

Last Seen: In Tenby, 24 October 2004.

Background: James was with around 20 other keen golfers on an annual trip to Tenby, West Wales. He failed to return to their hotel after a night out with friends, and his driver's licence was later found on the town's South Beach.

Ruth Wilson

Age at Disappearance: 16

Last Seen: Leaving her home in Betchworth in November 1995.

Background: Family raised alarm after she missed school; it was found she took a taxi to an isolated beauty spot.

Andrew Dill

Age at Disappearance: 38

Last Seen: 28 April 2003, at Hednesford train station, en route to his home in Birmingham.

Background: Andrew, a father of three, left no indication of his plans, but police have focused on Manchester, Wolverhampton and Cannock – as well as the Midlands area.

Paige Chivers

Age at Disappearance: 15

Last Seen: Leaving her Blackpool home, August 2007.

Background: Paige left home with a packed bag. Police have followed up sightings – and the possibility she may have joined a travelling fair.