When you get sexual services.
I think in most places they film / have a camera/ a secret mirrored window etc to ensure the safety of the hookers, ie so they don't get raped/assaulted by some loon.
That really sucks if they do. I hate the idea of being watched.
On the Cleopatras website in Ipswich they say they do on their own website, at least they are honest about it. I bet others do this and don't tell you they do it.