It is every person's - particularly every parent's - worst nightmare. For a loved one to walk out through the front door and never to return is one of the most heartbreaking, terrifying and harrowing experiences someone can go through. Not to know the fate of a person close to you is simply agonising - did they choose to disappear, were they involved in an accident or did something even worse befall them? Every day in the UK, a staggering 600 people...