As for the OP's topic, I do think it's a good idea to discuss what to do in this situation. If we crowdsource ideas in advance, it might help somebody else out someday.
Assuming the keys don't have any sort of address or ID attached to them, I don't think they are likely to get stolen. For that reason, I would be inclined to leave them near where I found them and very visible. On a fence post or picnic table or something.
If I took them elsewhere, like to a police station, I would leave a prominent note.
When I found someone's cell phone one time, I looked in the contacts for "Mom" and called her :-)
It probably wouldn't have occurred to me to post here, but I think it was a good idea!