Oh hey I've lived this one, kinda. I mean, I'm not single, but I am friends with a couple where I've hung out with one spouse while the other one was away. And so, we had told the other spouse beforehand that we were doing this. One time we spontaneously had dinner and told the spouse right away, as part of the spouses checking in with each other like they do. I think the husband should have told the wife before/during hanging out with the friend, not Because OF Course This Is Suspicious but just because people with interlinked lives should keep each other appraised.

If I were hanging out with this friend and found out they hadn't told their spouse I'd feel weird about it myself -- I wouldn't want the spouse to feel like maybe they couldn't trust me after all. But then I probably would have told their spouse by then anyway, since the spouse is also my friend and I think "I'm going to be in your house while you're away" is a sensible thing to let my friend know.

So, more communication, everyone! And texting totally counts as communication! (WTF was that part of the response).