This is why I do not use a debit card, cause its your money, not just credit.
They probably had a skimmer installed on the pump at the gas station. Down in South Florida, detectives found dozens of skimmers in self serve gas station pumps. It takes your card, and send the number to someone nearby. They either use it themselves or sell the numbers on the black market. If they sell the number, the person who buys it often will charge a small amount to a charity to make sure the card is good and something like that wont raise any flags with the card company.
This is why its important to check the statements every month and run a free credit report at least once a year.