It'd be interesting to see how this might shake out related to lost-and-found precedent - I bet there's some insight there. If you find a sweater abandoned in the woods and take it home I'd assume that isn't stealing unless there was a reasonable case to be made that the article of clothing was recently discarded - but if someone comes looking for it or puts up fliers that may change the math on that.
This is essentially the same idea - something was left on your car, potentially maliciously even.
It's an interesting point, though I wonder how the intentionality would play into it. Malicious or not, it's hard to argue that someone who places a tracker (or a bomb!) on your car has "lost" it, as opposed to knowing exactly where it is and wanting it there for specific purposes.
This is essentially the same idea - something was left on your car, potentially maliciously even.