The front tire.
Rear tire was at 24 PSI, right where I'd left it; the front tire was totally flat, so flat it didn't even register on the pressure gauge.
I'm not kidding: this is insane. The bike was inside a locked garage and the tire that hasn't been giving me fits was the one that went flat while the one with the intermittent leak retained all pressure.
What the hell is going on here?
...so I'm starting to think I'm going to need a priest to exorcise the thing, and maybe get a gremlin bell, too. WTF.