If there are kids kicking a ball around and it lands in your yard, are you legally obligated to give it back?
If it landed on my property they'd get it back, eventually. I wouldn't make it easy for them, though.
But that's just because I'm "bad".
It would be nice BUT You can't keep it. You must do your best to contact the owner or the authorities. Some people think that if the Post office delivers something you didnt order to your address that they can keep the item. That's theft and the...