Good for you, but depending on where they got/saw your address, someone might be trying to launder it...and they may not be able to receive it back at the original address it was sent from, anyway.

Hold onto it until someone contacts you, I would say. Don't try to cash it out. That way if law enforcement sees it and gets ahold of you, you can provide them with contact details from whoever is looking for it. It certainly sounds suspicious and like someone is trying to take advantage of your goodwill somehow to launder it.