On the evening of April 6 this year, John Halford packed his suitcase and left it outside the door of his cabin on the cruise liner Thomson Spirit. It was the last day of a week-long Egyptian cruise and the ship was due to dock at Sharm-el-Sheikh the following morning.Mr Halford, 63, texted his wife Ruth, who was at home in Britain, to say he would see her at the airport the next day, then went off to dinner. At about 12.30 am, he was seen by other passengers drinking cocktails in an upper-deck bar. He then vanished.Mrs Halford, who has three children, Lucy, 20, Sophie, 18, and Connor, 17, learned of her husband's disappearance as she was getting ready to drive to the airport to collect him.'The phone rang, it was the Thomson's desk at the airport in Egypt,' she said. 'I was told the plane was in the air but my husband was not on it. He'd gone missing from the ship. You could have knocked me over sideways. It made no sense. The children and I were shell-shocked.Today, more than five months on, Mr Halford, a bookseller from Milton Keynes, Buckinghamshire, remains missing, his fate unknown.Cruise ship holidays are enormously popular. According to the Passenger Shipping Association, 1.7 million cruises will be taken in Britain this year (many will be repeat cruises by the same holidaymakers). But what is happening to all these passengers who simply vanish while at sea, never to be seen again?Are they the victims of a sinister crime wave? Have they had a mishap at sea and fallen overboard, or perhaps chosen to take their own lives?The sad fact is that, in many cases, no one knows. And for the family and friends they left behind, that only compounds the heartache. Loved ones such as Ruth Halford and her children, who remain in limbo; bereft, baffled and unable to grieve.'John had been really looking forward to the cruise,' says Mrs Halford.'He'd once worked in Libya and was intrigued by North Africa. He was fascinated by ancient Egyptian culture and wanted to see the pyramids.'He went alone because we couldn't afford to go as a family, plus the children had exams coming up. Ships are places where it's easy to meet people, and John didn't mind going on his own. The passengers who saw him in the bar say he was not drunk and was in good spirits.'He'd packed his suitcase ready to go but his other belongings - his passport, glasses, mobile phone and rucksack - were found in his cabin. But there was no sign of John.'John wasn't depressed - there was no sign at all that he was contemplating suicide. He just wasn't like that.'His suitcase was later returned to us and in it were three necklaces for me, Lucy and Sophie with our names written in hieroglyphics and a similar name bracelet. John was planning on coming home to us.'At first, Mrs Halford, 46, believed that her husband, with whom she was about to celebrate their silver wedding anniversary, would turn up. But, as time has passed, her hope has nearly all gone.But is the idea of someone 'slipping overboard' credible? The rails on cruise ships are at least 3ft 6in high, which makes it incredibly difficult for anyone - even someone who might be drunk or ill - to pitch overboard.With no clues as to where or how her husband vanished, Mrs Halford is struggling to rebuild her life. After taking time off work after John went missing, she has now had to return to her job as a medical secretary to pay the bills and support the children.'Life goes on,' she says. 'I need money to pay the bills and we've lost John's salary. John took out travel insurance and I've been on to the company to try to make a claim but they simply say: "What are you claiming for?"'Thomson haven't given me any support, either. John was in their care, but I haven't had so much as a letter from them. I can't get a widow's pension because we don't know if John is dead.'We're living a nightmare and we can't see a way out of it. It is so unreal that we can't grieve. We are in limbo. What do we do? Should we hold a funeral? But how can we if we're not sure he's dead?'Last Monday, Mike and Ann Coriam met MP Mike Penning, the Under-Secretary of State for Transport, who has responsibility for maritime issues, to discuss a change in the law that would allow UK authorities to investigate cases of British nationals who go missing on vessels while abroad.At the moment, investigations are handled from where the ship is registered.Miss Coriam, from Chester, vanished as the Disney Wonder was on passage from Mexico to Los Angeles. A single policeman in the Bahamas, where the ship is registered, is investigating her disappearance.Miss Coriam was working on the ship as a youth activities coordinator. She was last seen by a work colleague at around 5.45 am on March 22 by a male colleague and appeared upset, but when he asked her if she was all right, she said she was fine and on her way to bed.John Jennings, Rebecca's uncle, says: 'Personally, I feel that someone has done something bad to Becky. The police officer who came aboard the ship to investigate concluded soon after she went missing that at that time there was no sign of foul play, but I don't share that view.'The implication was that Becky had committed suicide, but there is no indication at all that she was depressed. Yes, she'd had some sort of argument, but it could have been over something quite petty.'Rebecca had bought four tickets for Disneyland Paris as a surprise so that she could go there with her mum and dad and sister Rachael when she got back.'That's not the action of someone contemplating suicide. We have discovered recently that someone has changed the password on Becky's Facebook account, so we can't get into it. It must be someone who knows her, who knows what the original password was.'Her disappearance echoes another mysterious case that has been barely reported in this country - that of a 62-year-old German, identified only as Sabine L, who vanished from Cunard's prestige liner, the QE2, in 2007, as it sailed off Madeira.Sabine and her husband Ludwig boarded the ship at Southampton on December 17, 2006, for a two-week cruise to the Canaries and Madeira.One night, the pair went to bed at about midnight in room 5167. The next morning, when Ludwig awoke, his wife was not in the cabin. She was never seen again.Had something sinister befallen her; or could she be among those who are suspected of having taken their own life while on a cruise ship?Just this week, there was precisely one such incident. Passengers lined the decks as the Balmoral made its way up Southampton Water on Tuesday morning following an eight-day tour of Norway's fjords.The passengers were preparing to disembark when the announcement was made. A passenger was missing. No one was permitted to leave the ship until the police, who were waiting at the quay, had completed an onboard investigation.Francis Hemsley, 89, of Walton- on-Thames, Surrey, had last been seen at dinner at 9pm on Sunday. Some time between then and the next morning, as the ship headed south along Britain's east coast, he'd vanished.A subsequent police investigation established that in this instance there was a note left in Mr Hemsley's cabin. Although they will not reveal what it said, Mr Hemsley appears to have decided that he wished to die, and wanted to spend his final days amid the serenity of the Norwegian fjords.No body has yet been found, and officially the investigation is ongoing, but according to Hampshire police, who are investigating his disappearance, 'initial inquiries indicate the man fell overboard and that it was a non-suspicious death'.His disappearance echoes that of another elderly Briton who vanished from the same Fred Oslen ship, the Balmoral, last October.The 79-year-old unnamed man had been on a three-week cruise in the Adriatic with his wife. She woke in the early hours when the ship was about 45 miles south of the Lizard in Cornwall and saw that her husband was missing.He had last been seen onboard at about midnight.After she alerted the crew, and a search of the 43,000-tonne vessel had established he was not on board, an air-and-sea search involving British and French coastguards and two warships in the area was launched.Nearly one year on, there is no body and no answer to the question the family keep asking themselves: what happened?Two eerily similar disappearances, then, on the same ship.Certainly, the fate of a 50-year-old woman who was a passenger on board the Sea Princess as it cruised the Caribbean last December, between Curacao and Grand Turk islands, gives credence to the 'suicide at sea' theory.CCTV footage shows the unnamed woman, who was on holiday with her husband, climbing over her cabin balcony in the early hours and falling into the sea.Her body has not been found, but at least the woman's family know that she chose to take her life and that she is gone.For John Halford and Rebecca Coriam's families, however, and scores of other people around the world, there is no such resolution.Thomson Cruises, the company Mr Halford went on holiday with, says it has been assisting Thames Valley police and the Foreign Office in Egypt with their inquiries, and acknowledges that it may not have given Mrs Halford as much support as it could have done.'We are sorry to hear Mrs Halford feels she has not had our support and we would like to apologise,' says a spokesperson. 'This is a very rare occurrence and we have learnt lessons from it.'But Mr Halford remains missing, as does Rebecca Coriam and countless others.Precisely what happened to them all are mysteries that look unlikely ever to be solved.