DeKok and the Lost Child

It is New Year's Eve and De Cock is enjoying a relaxed night with family and friends when there is an unexpected visitor. At his front door stands a scruffy looking man with a straggly beard who tells De Cock that two young men are out to murder him.
De Cock suspects he is one of Amsterdam's deranged people who live on the street, and he directs the man to the police station.
A few days later the man is found dead on a bench on the Noordermarkt square. He was killed. His name appears to have been Joop Koedam, a burglar whose name De Cock actually recognizes from long ago. Soon after there is a second dead in the same circle of the long gone underworld.
De Cock does not believe in a coincidence, and he feels the two men might be the victims of a criminal feud?