I spent the rest of the day grumbling about this. When you pick up someone's wallet, you're taking responsibility that you don't need. I had to search for the guy's phone number. I eventually found his girlfriend's mobile number in the wallet and left a message, knowing that it'd be really annoying for him if he cancelled every one of those cards. I left a note on the car nearest to the wallet.
This morning, the owner popped by the office to pick it up. The dude works for Nestle. I now have more free chocolate than you have ever dreamed of.