The BBC has more here
http://news.bbc.co.uk/2/hi/europe/4616264.stm
Excerpts:
Dutch police have launched an appeal for clients who visit prostitutes to report any concerns that the women are being coerced into selling themselves.
The Dutch legalised prostitution in 2000, but many sex workers still operate illegally - many against their will.
Last year, the Dutch police received more than 600 tip-offs about women who may have been forced into prostitution. Most of these women come from Eastern Europe and Asia.