Well it all depends on how the WS feels about the affair.
Many times affairs are cries for help in a marriage...or in the case of my cries for help for the terrible sexual abuse she went through as a pre-teen and teenager (see my story--
http://www.marriagebuilders.com/cgi-bin/ultimatebb.cgi?ubb=get_topic;f=31;t=009539 )
IN those cases yes they will leave clues.... unfortunately many BS's have blinders on thinking it could never happen in their marriage.
The best comparison the therapist made was regarding teenage kids that are doing drugs. They unintentionally leave clues because they are torn between the thrill of doing drugs and the moral conflict of knowing its not right and potentially dangerous for them.
My wife was "acting out" from a repressed memory dating back before we were married.
Clues varied such as taking photos from the hotel room she was at with OM that happened to be to a tourist destination. Showed me and the kids. Talked about how it would be a great place to go to on the next family trip. Told me about the OM having a vasectomy (dumb asked her how she would know such personal knowledge and she responded saying he told the other girls in their office were teasing him about it). Had him come by our house and borrow our camper for his family vacation. And many more things.
She was leaving clues indirectly trying to get caught. Yet she was still using a secret cell phone and aware enough to never call on the cell phone I provided her.
So she was both safegaurding the affiar yet still leaving clues.
Hope that makes sense.
Even some verbal things like telling me I don't have to worry about her having an affiar because she hates sex.....which she did even with OM but I never asked about affairs she just blurted it out one time.