The alarm. Not good for anything else, so may as well use it for that. Or pop the trunk if you can, and if it'll pop up high enough to be noticeable, and if you don't have anything in there someone might walk off with. Or that iPhone feature where it makes pretty decent guesses about when you're parking a car and tells you where it is.
In, say, a major shopping chain suburban parking lot, anyone annoyed by a car alarm going off for five seconds is just looking for something to be annoyed by. Different story on a city street or in a neighborhood, sure.
