I believe it all depends on the scanner system the stores use. The scanner systems detect RFID chips which are commonly destroyed when passed over that pad on store counters to disable the RFID protection on products. Depending on how sensitive or how accurate the security system is I believe they can pick up on radio frequencies from cell phones (And other devices... RFID key fobs for keyless car entry and so on) and set off the alarms. Here's a bit more on how RFID works, including store alarms: How RFID tags work - What is RFID and what are its benefits? Explain that Stuff!