One could use it for themselves to listen to if one found the t's voice soothing or wanted to review what happened in order to better take it in and other things like that - It would only need to be revealed if one confronted the t with the evidence. I do not see it as a betrayal at all if one was using the tapes for the former. Frankly I do not think a t would care one way or the other even when confronted with evidence.