I'm baffled at the moment.
Sat for hours with no leak. FW tank 2/3 full, which is more than it was when I first noticed the leak and drained the tank. Checking every 30 minutes to see if the leak returned, nothing.
Went back out to close up for the night, retracted the awning, and for giggles ran the kitchen sink water, cold and hot, ran the bathroom sink water, cold and hot, both for maybe 10 seconds or so. This is all on city water, mind you. Climbed down the stairs and sure enough the leak was there, but then stopped. It's not a running water thing, just a drip. But enough to see the fairly steady drips. Sigh...
Tomorrow I'll continue the quest to find the leak. Fairly certain at this point it's not the FW tank so that's a good thing, I guess.