DH found a set of keys near our house today. The road (but not house number) was attached to them.
I posted on our local FB page that we'd got these keys and asked the owner to PM me.
Jeez. I got answers ranging from "take them back yourself" (how, as I didn't know the full address) and "the owner may not have FB so will never know you've got them" and "give them to the village shopkeeper and the owner can ask in there.)
I wish I hadn't bothered if I'm honest.