K - I would wait on the talk. Wait and see what happens on his email after he gets home. It seems weird that he would erase ALL his work emails, why not just hers? Perhaps there is something funny going on with his email with him being gone. Either way, I would not tip him off that you know - keep this valuable source of information to yourself a little bit longer.
As for the voice-activated recorder in his car - aren't YOU the brilliant little superspy! I actually like that idea. lol
