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!
Happens all the time for me at two stores. Walgreens and Kohls. I like the looks I get when the alrm goes off as I'm entering the store. Had someone stop me at Kohls on the way out and I told them it's my phone. She said, "Ok, if you say it's your phone then I guess I have to believe you." And let me go. Good security.
My buddy had this happen to him with a different phone, but he had to be wearing his leather coat and have his cell phone in his pocket. we went through this with a door greeter at walmart. He took his coat off and passed it through the scanner and it went off. He then removed his phone and everything in the pockets and passed it through the scanners and nothing happened, next he passed everything through the scanner and nothing happened. he went through the scanner and nothing happened. after that he put his coat on, put everything in his pockets and it again set it off. This occured on several occasions and they only consistency was his cell phone and his jacket. something about the combonation caused the scanner to trigger, AHHHH modern technology isn't great, lol
This has happened to me so many times at blockbustger that the employees are used to it by now and don't give me a second look lol. It also happened to me at FYE xmas shopping. The girl and I got in argument over how I could steal something going IN to the store....*sigh*
I am never without my firearm. I have never had either the droid, or my firearm set off an alarm. However, I did have an alarm go off before for some other people at Best Buy. They insisted to search me and I would not allow it, showed them my police ID and they told me to continue on. I didn't want the high school kid to feel my gun and cause a scene...