It has happened quite a few of times with me, on all accounts they were with girls from incall places though, and mostly asian as well; rather, they were more like first/subsequent dates where you get lucky than anything else. From what I've noted, sex is almost always a part of it (if she likes you) when going out with an asian-bordello girl; however, with AMP girls, it's pretty much up to her own moral standards.
Whatever the matter, if you like a girl you've had a session/sessions with, and she seems to like you too; keep your ears sharp if she starts talking to you about a club somewhere or something, or, for Chinese girls, if she begins to talk about certain dim sum places; she just may be hinting for a non provider-client outing