I've had a couple of people raise PayPal disputes with the claim that their PayPal account was used in an unauthorised transaction - these are actually quite good, in a way. I don't mean it's good when they occur, of course, but they can be better than other kinds of disputes (more so if the buyer is being dishonest) because PayPal has the tools to verify whether the account holder's claim is legitimate in a few different ways, and they will use them.

The ones I had opened against me were all found in my favour because at the conclusion of their investigation, PayPal found no evidence that the accounts were compromised as the buyers claimed. None of them sent the item back, though, so I wasn't faced with this kind of conundrum.

I sometimed get unauthorised returns, as well - as in, the buyer just sends an item back without contacting me and the first I know about is opening the package. I nearly always have to go searching for matching transactions / details to find out who it was, too. It's frustrating, and technically I would be within my rights to deny a refund and just tell them if they want it back, they can send me the postage cost, but they've all been in as sent condition so I will usually not contact them and just issue a partial refund, of item price minus all costs incurred. If there were other aspects to the return that were weird, annoying or suspicious (eg pieces missing, etc), or it was outside of my stated window, I might handle things differently, and I'd also block them.