30 PSI. Actually just on the minus side of 30 PSI, which is a bit lower than it was when I checked it earlier--whenever the hell that was.
But 30 PSI is too high for riding, so I lowered it to the manufacturer-suggested 24 PSI and then took it out and rode it around for a while. Once done, I checked it again.
Put it away, let it sit overnight; and then this afternoon I checked it after bringing the garbage can back up.
All right--someone's got to be letting the f-ing air out of the rear tire when I park it outside and leave it there for a few hours, because the dang thing is not going flat when I scrupulously park it inside. It might--might--be losing a little bit of air in 24 hours, and maybe the variation is just due to the change in ambient air temperatures. I don't know about that; what I do know is that it's not leaking down to ambient pressure in 5 hours when I'm the only asshole who can get at the thing.
It doesn't make a hell of a lot of sense--why just the rear tire?--but WTF, if it were some shithead kid thinking he was playing a funny prank I guess I could understand it. Nothing else makes sense; how else could a tire with a leak big enough for it to go flat from fully-inflated in five hours somehow magically not leak at all sometimes?
The next thing for me to try? Taking it for a ride up to 55 MPH, and then check it...and take a can of Fix-a-Flat with me, or something else that can inflate the tire for me should it go flat while I'm riding.
Ask anyone who does troubleshooting--intermittent problems are the worst. *sigh*