i inadvertantly left a credit card behind in a shoe store once (encumbered by 3 kids at the time). whether intentionally or not, the clerk did not return it to me and i didn't notice it. til i got a call from Discover a few days later and they were calling because right then and there someone was trying to purchase $500 worth of tennis shoes at a sports store! they were very on the ball, and were the ones that tipped me off. I called the store and let the manager there know that this is what she did, and turns out she had not shown up for work on that day.
this was Mastercard and i was not impressed with their handling of this, since i would think that $20k worth of computer hardware would send up a red flag and prompt a phone call to the cardholder.
I remember getting a call from American Express one day, they were looking for my business partner. It seems his Amex card was stolen while he was checking out of a hotel in Harrisburg.
Amex knew about the carad being gone before he did and I knew it before he did - a very astute clerk in a sunglass store found it hard to believe a teenager would have a Gold Corporate Amex card and asked him for some ID - when he didn't return with ID the clerk called AMEX and they immediately called us. Fortunately I was on the account so they were able to speak with me about the problem.