More victims?

"And as he contemplates a life behind bars, I can assure him (Sweeney) that this investigation will continue as we seek to identify and trace other potential victims in the UK, Netherlands and elsewhere in Europe, who may have suffered a similar fate to that of Melissa and Paula."
Detective Chief Inspector Howard Groves, Metropolitan Police statement, 5 April 2011
In addition to the devastation of the families of Melissa Halstead and Paula Fields, police suspect there are other victims who have yet to be identified.
Within the artwork used to link Sweeney to the murder of the two women over three decades and in two countries, police believe there are images of other girlfriends of Sweeney who could also be victims of his.

Two of these women went missing during a six-year spell that Sweeney spent on the run from the police — a period during which he murdered Miss Fields.
The first is a Brazilian woman, called Irani, who frequented pubs and restaurants in Highbury and Holloway Road in north London in 1996 or 1997. Officers believe she worked in kitchens as a cleaner.
The second woman is thought to be a Colombian, called Maria, who was living and working in, or just frequenting, pubs and restaurants in the Finsbury Park and Holloway Road in 1997 and 1998.
A third woman officers are trying to trace is a British woman, called Sue, from Derbyshire. She lived in the North London area and was believed to be aged in her late-20s or early-30s in the late 1970s or early 1980s. Officers believe she was attending a nursing college and went to Switzerland to work.