I wasn't worried-- I figured I'd left the phone in the car.
When I went downstairs later to head for a poker tournament, I didn't see my phone anywhere. However, much to my surprise, someone else's phone was between the seats!
The battery of the stray phone was fully charged. The greeting that the phone displayed was "BUNNY WUV U", and I didn't recognize any of the numbers in the extensive phone book. This phone (a small Motorola) looked nothing whatsoever like my Treo palmphone.
What the fuck? And how would you go about sorting it out?
I'll put a detailed chronology of the day in a comment.