There is a difference between being a good Samaritan, and dealing with corporate stuff. If I walked into the OP's store and asked him for something for free, because I needed it, would my expectation be to get it? No. Would I make the comment of where is the store owners morals? No.
Last I checked, possession is 9/10ths of the law.
Is it the "right" thing to do, to return it? Probably. If I lost a phone or wallet or something traceable to me, I'd hope that the finder would return it to me. It is exactly what id do - return it to the owner via reasonable effort to find and contact them. On the other hand, i guess i dont have enough faith in humanity, so I can imagine they would also be annoyed if I didn't compensate them for their hassle and the time/expense they saved me. But this is also a bit different perhaps - why should someone give a freebie to an entity that exists to make money? Isn't business all about creating "value"? Is the return of the item creating value vs having to buy another?
Now on the other hand, if I lost my wallet IN the store, and the store called me because they had it, I'm no sure Id compensate them for it, unless it was a waiter or some other underling.
So it kind of is a double standard, as I see it.
I'd have either told the guy that he is in posession of my property and I'd be reporting him to the police if not returned, or else just say thanks, keep it and scrap it, we're reporting it's loss/theft to the police and so they will be on the lookout. At least in NJ, a DL is required to sell scrap.