But to say that keeping coins that magically appeared one night on my address with absolutely no agreement in place regarding the transaction, are not mine, is laughable.
Magically appeared? You know who sent it, you know that it was a mistake and you know how to return it.
If someone accidentally left a wallet in your house and then knocked on your door asking to get it back, would you do the same? "Oh, you mean this wallet that magically appeared on my living room table? Well, we didn't have an agreement regarding that so I think I'll keep it. Thanks!"