Performing actions based on the location of a mobile device during a card swipe

In some examples, a computer system may receive transaction information for transactions between card users and a payee that may include transaction card information from a plurality of read events of a plurality of transaction cards at the payee's card reader. In association with the plurality of read events, the system may receive location information of a plurality of mobile devices, each mobile device being associated respectively with one of the plurality of transaction cards in a data structure maintained by the computer system. For instance, the location information for each mobile device may be indicative of a location of the mobile device at a time of a corresponding read event of an associated transaction card. The system may determine an approximate location of the card reader based on the location information of the plurality of mobile devices.