My first assumption was that I'd accidentally left my car unlocked, somebody decided they wanted my phone, and accidentally dropped theirs in the process of grabbing mine.
To that end, I sent myself a text page that said, "Please return my phone. I have yours."
I then turned my attention to the strange phone. The greeting was "BUNNY WUVS U". Hmm... I looked through the address book, found two listings for Bunny, and called her.
"Hi! I found this cel phone in my car today. Do you know who it belongs to?"
"It's my boyfriend's phone. I thought it was him calling."
"Hmm. OK. Do you have any clue why it might have wound up in my car?"
"He's a valet who works for Ampco. Did you park leave your car in a garage downtown today?"
"Aha! That explains it." We made arrangements for them to come pick up the phone later that evening, and that I turned my attention to finding my phone.
I tried calling it both from the loft and from the car, but no luck-- I didn't hear it ring. I tried calling the Walgreens I'd been at, but they didn't have it either.
So I went off and played the poker tournament. When I got home that night, I opened the passenger side door for some reason, and there was the phone wedged between the seat and the door. Whew!
At first, I was sure the disappearance and appearance of the two phones had to be related. When all was said and done, though, it was just a strange coincidence.