How? Have you ever been through anything related to stalking or even heard anything about it? Stalking is a serious offense and what was done here was not that. The OP wasn't sitting there waiting for the therapist to come out or going through their stuff or following them home or anything like that. It's a curiosity thing and there isn't anything wrong with it, "IMO".

Anyways.. I wouldn't worry about it I don't see it is a big deal and as long as you've told your T and she is fine with it don't worry. She should have a good explanation as to what would cause you to have those feelings. Best of luck.