> Most large retailers only store the full credit card information for seconds while they process the transaction, when the transaction is complete it is discar

> Most large retailers only store the full credit card information for seconds while they process the transaction, when the transaction is complete it is discarded. Couldn't work around that by having some device that attaches to a randomly selected POS terminal, and then executes a series of dummy transactions using thousands of "canary cards" that were assigned to the store? They could perhaps set this up with the credit cards companies so any use of the canary cards are automatically flagged when they're used outside of the store that was issued them.