Found.

The prodigal cat has returned. Apparently he DID get out of the house, probably when the plumber left late yesterday afternoon (did I mention the sewage backups we've been having? no? well, add that to the mix), and Mr. Squab found him outside near the neighbor's deck. He seems to be just fine, his usual old complaining, grumpy self. Lord have MERCY I'm glad he's home. The cat gods would've cursed me for sure if I'd lost him.

Tomorrow's mission: getting some damn collars for both cats. (And yes, I know I'm a total asshole for not having done this long ago.)

Thanks everyone for the offers of help and good thoughts.