Originally Posted by PR_Tom

People who pay cash for small and expensve items, then return 10 minutes later for a refund without the receipt are always a dead giveaway.

What they did is steal the same item previously at another store (or your own if they're bold), and after your stupid employee refunds them with no receipt (they remember selling it to them a few minutes earlier for cash), they'll return the stolen one with the receipt at another location.