Two years ago I was contacted out of the blue by a retired police officer who asked if I remembered the case of Ruth Wilson, a 16-year-old girl who had gone missing from Dorking, Surrey, in November 1995. I told him that of course I remembered – it was one of the most peculiar stories I had covered as a journalist.

Ruth had left home as usual on a miserable winter morning, but instead of going to school, had taken a cab in the pouring rain to the top of bleak but beautiful Box Hill, where she vanished off the face of the earth. A good girl from a good family, Ruth has never been heard of since and no body has been found.

I first wrote about the case in the Observer more than 15 years ago and it has haunted me ever since.

In 2002 I concluded that the circumstances of the Ruth Wilson case were perhaps just too odd to become the focus of a media campaign. There had been some attempts to use her family to draw attention to the case, but they were not keen on publicity. Over the years there was a series of further appeals, but even in Surrey the Ruth Wilson story just faded away.

And it would have remained in obscurity were it not for Liam McAuley, a 58-year-old retired police officer from Northern Ireland, who came across the story in a local paper when he moved to Dorking in the mid-1990s. “Nobody can actually just vanish,” he told me. “I think something terrible has happened to her. Somebody knows where she is.”

McAuley made a freedom of information request to the police to ask for details of the interviews carried out after the disappearance. He was told the case was still ongoing and he could not have the documents he had requested.

He wrote to Ruth’s parents, Ian and Karen, who still lived in the village of Betchworth, near Dorking, but received no reply. In his frustration he contacted me as the last journalist to write extensively about her. His tenacity has led to the making of a documentary, The Vanished, released this weekend.

The circumstances of Ruth Wilson’s disappearance had always left me uneasy. Why had this studious, church-going, bell-ringing, choir-singing, organ-playing young woman suddenly decided to make a new life for herself?

In the intervening years, I had discovered something about the Wilson family they had chosen to hide from me at the time. Karen Wilson, introduced to me as Ruth and her sister Jenny’s mother, was in fact their stepmother. Their birth mother, Nesta, had died in tragic circumstances when Ruth was a toddler and Jenny still a baby. Why hadn’t they told me this? Was this really such a happy family? And did the death of her mother have anything to do with Ruth running away?

When we started making the documentary Ian Wilson got back to me saying he and the family didn’t want to participate, although he trusted us to do “a professional job”.

With the family refusing to speak, we decided to contact Ruth’s friends. We knew she had recently split up with her boyfriend, Will, and I managed to track him down to the south coast. Will, who does not want to be identified, explained that Ruth was a troubled teenager. She was unhappy at home. Will confirmed that her mother had died. The story he had heard was that she had fallen downstairs and broken her neck.

Following an appeal in the local newspaper, other friends came forward. Roxy Birch, a schoolfriend who played Ruth in an early reconstruction of the disappearance, told me: “She couldn’t drive, as far as I am aware, she didn’t have a passport... So, you have to ask yourself the question, where could she have disappeared to for 22 years?” Kay Blenard, another schoolfriend, said: “My belief is that she had planned to do something. I don’t know whether that would be permanent or temporary. I’d also like to believe that someone knows what happened.”

Nesta Wilson’s death certificate showed that the story of the accident on the stairs was not correct. The awful truth is that Ruth’s mother committed suicide: she hanged herself just before Christmas in 1982, when Ruth was four and her sister a few months old.

Just after receiving this grim information, I was contacted by another of Ruth’s friends. Catherine Mair grew up close to her in the sixth form of The Ashcombe School. Crucially, Catherine revealed that Ruth had found out about her mother’s suicide just before she disappeared. She was devastated. “Ruth was really troubled,” she said. “She had so much going on in her head that she was desperately trying to find out who she was. ”

We put this to the Wilsons and they issued a response. “Her family are extremely hurt by this statement and do not recognise this view of Ruth’s childhood,” they said. “Ruth always knew about her biological mother’s death, but not the exact cause. Sadly, we now know that before her disappearance, Ruth had discovered the tragic circumstances of her mother’s death, but equally sadly, she chose not to discuss or question this with any family members.”

Jon Savell, the chief superintendent, public protection, at Surrey police, carried out the latest review. “There are five explanations for Ruth Wilson’s disappearance,” he said. “A tragic accident, abduction, suicide, murder, or that she had absented herself to start a new life.” I asked him whether the police had known about Ruth’s unsettled home life and her birth mother’s suicide. He confirmed that they had indeed known about the family background, but chose not to make it public in case it coloured the testimony of any witnesses who came forward. So it was that the narrative developed of the perfect middle-class home and an inexplicable disappearance.

There are so many unanswered questions. Why did Ruth send her stepmother flowers to arrive two days after her disappearance? Where is her mother’s family and why did they not come forward? And why, if she ran away, has she never made contact with her family and friends?

Someone knows what happened to Ruth. And secrets have a tendency to come out in the end.

Vanished: The Surrey Schoolgirl can be seen on the Real Stories YouTube channel