I found one once. It wasn’t locked so I looked in the contacts, found and phoned an obvious family member of the owner. She said it had already been reported lost so she didn’t want it back. Shortly after that it was locked by the operator.
Assuming that it belonged to the network operator, I sent them an email, explaining the circumstances, asking how I should return it to them.
I suppose I expected thanks and an offer to refund my postage – I got an aggressive response that implied I’d stolen it. I decided it wasn’t in my interests to give them my name and address, and I wasn’t going to spend my money posting it to them if that was their attitude. I don’t know where it is now.