I went back in, got undressed and went back to bed, thinking I'd check it more in the morning. Of course, I couldn't fall asleep until I checked it again, so I got up, got more dressed this time and went outside. I checked the truck and couldn't find anything wrong. The hood was up a bit, but the latch isn't working right and it is usually up. I pushed it back down and went to bed. Strangely, this time I was much colder than after coming in the first time, though I spent roughly equal amounts of time outside and was more dressed the second time.
Of course, I couldn't stop thinking about what had set it off:
Someone bumped it with their car (shared driveway)
Someone tried to open a door (never had that set off the alarm before)
Someone dropped something on it from upstairs (nothing seen, but the neighbor was out there too)
Someone was trying to steal engine parts (never had that set off the alarm before)
Someone was planting a bomb in the truck's engine! (ok, now you're just being weird, go to sleep!)
So I still have no idea what caused it. I'll just hope it isn't going off all day.