If they found it in your garage? Get serious.
To me the ambiguity is created by the fact that the finder has no way of knowing for sure if you will get it back if they turn it in.
So what if I find it and just decide to leave it. Does that make you somehow a bad person?
I am not obligated to you in any way by the act of finding.
Returning it certainly might make me a better person, but not returning it does not make me a thief. Of course we obviously aren't going to agree about this, so I will just let it go...