Jason Bennetto, Crime Correspondent, went to Scotland to try to unravel the mystery.
Roddy and Ann Marie Aitken seemed to have a lot to live for. Christmas was coming and they were about to spend two weeks with their 13-year-old son, Roddy, in Cyprus catching some winter sun.
They were, says their other child, Ann Marie, who turned 20 this week, loving parents who thought the world of their family. On 18 December they drove the 25 miles from their home in the tiny town of Aberfoyle, in central Scotland, to a Glasgow suburb to deliver Christmas cards to Roddy senior's parents, Jessie and Archie.
Shortly afterwards they spoke to their daughter and a friend on their mobile phone to say they would be returning in an hour or so. Ann Marie recalled: "They said they were still doing some shopping and wouldn't be back for a while. We didn't talk long - mobiles are so expensive. I got worried the next day but I didn't want to telephone anyone in case they just turned up.
"I kept thinking they would be back tomorrow. I thought they must come home for Christmas Day. My wee brother went to his friends for Christmas. I sat here with my boyfriend and waited for the phone to ring." She waited until Boxing Day - eight days after the disappearance - before her grandparents persuaded her to get the police involved. Speaking at the family home, where she is staying with her brother, she added: "I don't think they will be coming home now, it's been a month without a trace - it's just too long."
For the past fortnight a huge search operation has been taking place among forest-covered peaks and along miles of twisting road amid some of Scotland's most stunning countryside. Interpol and police forces throughout Britain have also been alerted.
But so far no bodies have been found, no burnt-out wreckage discovered, no tell-tale skid marks or car roof staring up from the bottom of a loch.
The Cyprus holiday, which had been paid for with pounds 1,000 cash and was their first foreign trip in six years, remains unused, and the couple's bank accounts are untouched.
Rumours of underworld connections and contract killings have surfaced but detectives believe they are fantasy.
The police are baffled. Surely the bodies would have been found by now if it was an accident, and why would they run away at Christmas? Was this a murder or missing-persons inquiry? Were they looking for one or two people? Nothing makes sense.
As part of the detective work the hunt for the two 38-year-olds shifted yesterday to London, where the family lived before moving to Aberfoyle seven years ago. The six-year stint down south in Fulham and later Sutton, in Surrey, where Roddy worked as a roofer, was the only time the family ventured out of Scotland.
Brought up amid the violent, sprawling Glasgow council estates of Drumchapel in the north-west of the city, Roddy and Ann Marie started dating at school, were married at 18 and parents a year later.
As a child Roddy was taken to the Trossachs, a wild, beautiful land of steep- sided glens, snowy peaks and dark lochs, about 35 miles north of the Glasgow estates. When a chance came to swap their London council flat for one in safe and unexciting Aberfoyle, Perthshire, he grasped it.
But the Glaswegians did not fit into the small, close community. For one thing the parents did not work, relying mainly on benefits, and a disability allowance from a car accident in which Roddy injured his leg. Then there was the drinking. Roddy was barred from several pubs in the town after a number of alcohol-fuelled fights and arguments.
Neighbours said that they were "friendly enough", "no trouble really", "kept themselves to themselves", while one commented: "They were still quite new - seven years is a short time in Aberfoyle". They did most of their socialising in their old Glasgow haunts. On several occasions Roddy drove home along the back roads after having one too many.
In the past fortnight hundreds of miles of forest tracks have been meticulously searched by the police in case their blue Vauxhall Astra slipped over the edge on that December night.
So far the only clues are a 7pm sighting of them at an Asda store just outside Glasgow shortly after the visit to Roddy's parents, and the calls to a friend and to their daughter.
Shortly after the police went public a tabloid newspaper claimed the disappearance was linked to underworld connections. Police investigations have shown this is almost certainly nonsense. Roddy was apparently considered a "Ned" - a ducker and diver. But he was not known by Glasgow's criminal community and with a handful of convictions for minor drink-related offences it was hardly the stuff of The Godfather.
The officer in charge of case, Detective Chief Superintendent John Ogg, said: "There are no indications whatsoever that there are drug barons or underworld enforcers involved."
He added: "There seems to be a piece of the jigsaw missing. We feel there is someone, or some people with vital information they are withholding."
So are the police looking for a killer or runaways or both? As one officer indicated, we may never know: "This area is very hostile for anyone looking for a body, especially if hidden. You could search for years and find nothing."Reuse content