Sure you can look in the phone book but if you can't even tell me this person's name then why should I give you their number?
Because he found a set of expensive, electronic keys that someone is probably severely upset over missing, and the only clue he has to their identity is their Club Card number?
In addition, he first requested to simply drop the keys off at Vitamin World and have them contact the person, which is not something a scammer would do, since it's too likely to backfire.
Remember, that unless you go with the idea that he's a lying bastard, the story he's giving is that he's trying to do a good deed. he also already has the card number, which is solid proof that he is in physical possession of either their entire wallet (which would have ID as well) or their key-chain.