I left an envelope on a shop till which had an address and a stamp on. It was very thin and slightly see through so obviously no cash or anything worth stealing.
The shop said they haven't seen it so I guess someone picked it up with their shopping.
Would you post it, if you found it? In all honesty, I'm not sure if I would because it seems somehow wrong to, as if I'm taking responsibility for someone else's decision.
In my case, it doesn't matter if they do or don't but the recipient and me are curious to see if it turns up.
YANBU - I would NOT post it.
YANU - I would post it.