Not nude but...
My wife and I go to a particular beach a few times every summer. It's far enough away that we don't really think to run into anyone we know. Sometimes she'll go topless. It's technically topfree, though almost no one does.
One day we went, and while she wore a pretty conservative bottom, I persuaded her to wear a tiny mesh see through top. These pictures don't do it justice, in person you can totally see everything. When she'd lay on her back, a boob would occasionally pop out. Normally not an issue for her.
Unknown to her, an ex-coworker of mine told me he'd be there this day. I told him to let me know when he was around, and I'd let him know where we were camped out. So he arrives and I explain our location. My wife is totally oblivious as he comes walking up to us. I greet him out loud, and my wife opens her eyes and goes wide eyed. To her credit she doesn't make any move to cover up. He shakes my hand, and gives her a hug. During the hug she gives a look like "did you know???". I just smiled back.
We spent about 1/2 hour catching up, as we hadn't seen each other for a while. He'd never seen my wife in anything other than normal clothing, and I could see him taking a glimpse every once in a while. How could you not? Again, she made no move to cover up, and in fact relaxed and became pretty comfortable.
After a while he said he had to get back to his own family, handshakes and hugs again, and he was gone. She looked at me and said "You knew he was here!". I just smiled and told her I enjoyed that
We've had some topless run ins as well. I'll tell those stories later...