I found that someone had gotten into it.
Not broken into it; turns out I'd left a door unlocked, and in the couple of weeks since I last drove it, someone got in. They clearly had looked for something valuable; the change door (which had been empty) and the glove compartment were both open. Stuff from the glove compartment was spread around the passenger side seat, but it looked like nothing was missing. My hand warmers, gloves, maps, and insurance/registration pack were still there with the appropriate paperwork.
A little dispirited, I cleaned up, feeling lucky that nothing (as far as I can tell) is gone. This has happened only twice in my time in Portland; the other time, at my Brooklyn neighborhood apartment building, the person who got into the car took some (not all) of the middling amount of change I'd had in my car, and even left behind (lost?) a glove. That person probably lost money.
Meanwhile, after this time, the car is home...and locked again.