We have post office box. We found some important mail in the lobby trash addressed to us. I asked the post master about it. He said he must have put it in the wrong box. Once someone else receives it, it belongs to them.
It seems that what they tell you depends on the situation and if they're in trouble for goofing up.
LOL. Right - so if someone's paycheck arrives in your box by mistake - it's yours? Or if someone ordered something out of a catalog / online and it gets delivered to you by mistake, you get to keep the products? LOL.