I could be wrong as I have little experience with the category, but I am under the impression that they are expected to maintain confidentiality with their clients, in a similar way to lawyers, doctors, priests and psychotherapists.

So if A hires Mr. Bogart to spy on B, and then C comes to Mr. Bogart and tells him "I'm interested in B, and a little bird told me you spied on B for someone", basic professional ethics would require Bogart to refuse to discuss anything related to A's case with a random stranger, potentially costing him his licence should he fail to do so (depending on what regulations apply in Bogart's country).