Sounds like a kid. Really. children do illogical things, like break into a car that doesn't look like it has anything in it, just because they can. Also this would explain the stealing of shiny thing ("Oooo, a knife!, cool!") and not taking much more valuable inverter. The seat moved forward assists the hypothesis (smaller person), as does the drinkbottle and wrapper. (You could get the DNA off it and track them down, man! ... Unless it's a child without their DNA on record, of course, then all you could do is have the DNA on file for when they commit some crime in years to come, and then you get a telephone call from the police telling you they caught the person who robbed you all those years ago.).
A friend of mine had his Accord broken into. This accord was a decade old and had been around Australia a few times, so it wasn't special in any way ... except for the sound system. They guy took it, but had to leave the amp under the driver's seat, as it was huge and the guy was probably disturbed. (He left his cigarettes.) Anyway, a few days later, the guy obviously came back, because he broke in and took the amp, AND his cigarettes.