I know snooping is against Sandi's rules, but at least for my situation, it led to me standing up for myself. I was content with GALing and being positive around W while I assumed she was not in contact with OM and was trying to figure things out. How it will contribute to the end of my situation, I don't know, but finding evidence finally gave me the courage to put my foot down, be a man and establish a boundary similar to you.
This was my experience as well. If continued contact with her OM was a dealbreaker for me (and it was), and on the other hand if I was to be loving and giving and FORgiving and even a little PURSUING in trying to reconcile . . . I personally saw no OTHER way than to independently get some intel on whether or not the affair was still going on.
To be, it was critical info necessary in my potentially life- and family-altering decision-making.
