I think some people may have received Bitcoin in their wallets from unknown wallets. As for me, I've never experienced that.
However, if you find Bitcoin in your wallet sent from an unknown wallet, you should find out who the sender is and return it to them. After all, it's not your right to hold the Bitcoin and claim it as your own. Even if you can't find the sender, you have no right to use it since it's not yours, so you can leave the Bitcoin in your wallet until the owner can contact you.
You don't need the sender to send back th Bitcoin, if someone send you Bitcoin and you know nothing about such transaction, it's either you leave the output unspent, don't touch it. If you are very familiar with coin control and address reuse, this is the best time to put it on good course. However, if you feel like returning it is the best option, send back to the same wallet address that send you the Bitcoin and anyone that controls the key will get the Bitcoin.
I will not be morally concern when someone send Bitcoin to me, that word doesn't exist on the network, I control the key and not anyone. Most often when Bitcoin is send to you, it's usually means to track you or involved you in a laundering scam. I mean, you can never see a person do mistake and send Bitcoin to wrong address, the only exception time this happen is when scammer makes victims download malware that's the likelihood of wallet address changes happen and Bitcoin will be sent to scammer. If you receive Bitcoin by any chance, you should be scared of spending that coin for your privacy and security reasons.
