It really depends on the details.

If somebody that you can't contact left it behind, I say keep it.
If the company doesn't acknowldedge it's existance, or has written it off due to the broken screen i'd keep it.

If it belongs to somebody who works there, or this person on HAN is a threat, your job is not worth it.

As for the Walmart thing, be smart about it. Take it to a store thats not exactly close just in case an issue were to arise.

You could always play it off like you were exchanging it for the owner :hsdunno: