More than 100,000 people are reported missing in the UK each year, and most are found within 24 hours.
But 30 years after Louise Kay, 18, disappeared after a night out in Eastbourne, what happened remains a mystery.
At the time, serial killer Peter Tobin, currently serving life for the murders of three women and previously jailed for rape, was working at a local hotel.
Could he provide the key to what happened on 23 June 1988?
In the first of a three-part series, former detective Mark Williams-Thomas looks into Louise’s case and others he thinks are linked to Tobin and to Angus Sinclair, who raped and killed two teenagers in 1977.