You said that her house is out of your way and inside a neighborhood. I feel that its intrusive to drive by her house. If it was on your way home or on your way to an errand maybe that might be different but you were actively seeking to drive by and see it. I think therapist deserve there privacy. I understand the need/want to know more about her and to see where she lives. I have wondered what my T's house is like but its another story to try and find her house and drive by.

I would not tell her. Its not worth the risk of loosing her. If you want to ease your guilt you could just not do it again. Of course the choice is up to you but I think you should really think about everything before deciding to tell her.