I found a wallet and left a note in a way so only the real owner could call me (old.reddit.com)

I once sent my credit card number to Cliff Stoll by sending 2 16-digit numbers from 2 email addresses to 2 email addresses. The sum was my CC.

I see some posts of lost items / wallets on local sub reddits. There's always the question of how much info to give and such but as far as the wallet situation goes this is pretty neat.