In terms of hard evidence, not much, but enough:
1. A text from OM on her cell. ("I miss you, my love", etc.)
2. Lots of calls/text from OM's name on her cell phone record until two weeks ago, then stopped suddenly and then two other names appeared (including the one she hung up on twice in my presence in the car).
3. An email in her trash that she hadn't cleared from a month ago to OM asking if he might want to come by, snuggle and watch a movie with her. Which happened to be the day after I left town on a four-day business trip. (I found this when she forgot to log off her account and fell asleep. A dagger through my heart.)
This is enough objective evidence to make me certain I am not crazy or jealous or imagining things. But the pattern of furtive conduct -- hiding in the bathroom with the door shut too often, locking down and jealously guarding her cell phones, unexplained or inconsistent absences or lapses, finding total fault with me, being indifferent to my well-being (other than formulaically), not being engaged in our home renovation and vacation plans -- is actually a bigger sign.
I need to find a better paper and voice trail to back it up.
Oh, by the way, I have confirmed that OM is married, but "unhappily". He has known my wife for many years and is a "peer" of hers in the profession. He lives in another city a few hours away but comes to work in the same relatively small office as she is in two days a week (stays over a night or two, without his family, of course). So he's a perfect fling for her: he's available every week, "understands her", and is really eager to find some lovin'.
I have never met him, but I hear he is a real animated guy. I think he gushes all over her. I don't "gush", but I am a warm person who frequently expressed my love and appreciation for my wife. I think she is mistaking flash for substance here.
I can almost hear what she's saying to him: "Oh, OM, you are so fun and easy to talk to. You understand me. We can talk about everything. And you're so hot! I can't wait to get more! I can't talk to my husband. He's reserved. He is a stick-in-the-mud, is only interested in [whatever]. And he argues all of the time with me. I think he's defensive about everything. I actually think he's screwing his [secretary/dentist/neighbor/whoever] on the side, you know. But only in the daytime, 'cause he usually falls asleep quickly at night. How fun is that?"
Let's mark this prediction and see how right I am about the dialogue when I find out more.