I have put a plastic name disc ring on to my keys with my telephone number on it then if you do leave your keys anywhere someone can soon ring you for you to collect them.
I decided to do this when one day I was at a boot sale which was crowded. I was just going to walk away from a stall when a man said. " Have you dropped these?" Gosh if he hadn't have picked them up and given them to me I would never have found them in the crowd.