We spent a couple of hours worrying, checking known hiding places, but no Gilbert. Marty went down to the basement to look around, but there are so many nooks and crannies down there, and not much light, so it's not surprising that he didn't find anything. He left some cat treats on the top of the basement stairs, a trick that's worked before. We started talking about whether we'd be able to leave tomorrow if he was still missing.
I was lying in bed, and suddenly Gilbert appeared and jumped up on the bed like nothing was wrong. I have no idea where he had been; it was as if he had suddenly materialized. So we have no idea where his hiding place is; we're just very relieved that he's back from wherever he was.