It was Brake Light Circuit.
All my brake lights were working.
And, it went away once I shut the car off (didn't come back one when restarted a few minutes later). It was still raining a little bit when it happened, so I'm thinking you are probably right with the condensation guess. Anyway to seal it off so moisture doens't get in there?
I'll keep my eyes peeled to the clock the next time I hear those beeps, too.