In Germany, the Extra Future branches of the gigantic Metro Group $($department stores$)$ were forced to withdraw its customer loyalty cards when the press discovered that the company had secreted radio$-$frequency identification $($RFID$)$ tags in the plastic. Even when enclosed in a wallet or purse, these cards could track which departments the customer visited. The retailer was hoping to engage in $\_\_\_\_\_$ and look for unknown shopping patterns.