Detectives investigating the disappearance of a young mother more than 40 years ago are extending their search to a residential area in West Dunbartonshire.br br Mary Duncan was 17 years old when she disappeared from her home in Alexandria in 1976.br br The teenager, who had a 13-month-old daughter, vanished after telling her family she was going to meet a friend.br br Her daughter later died of natural causes.br br No trace of Mary has ever been found despite extensive inquiries.br br On August 17, police announced they were searching a number of sites at the Vale of Leven Hospital in Alexandria.br br Detectives have extended the search to include a residential site in Helensburgh, which will commence on Monday.br br Detective Superintendent Calum Young, from West Dunbartonshire CID, said: "This latest development is due to extensive enquiries carried out in an attempt to find any indication of where Mary could be.