Phone baseband proprietary stack is bombarded with type0 SMS which will silently ack the msgs and thus 'ping' your location. There's probably carrier made tracking programs on SIMs too and/or your SIM can be modified by over the air update to install stealth tracking programs the host OS will never know about. FBI asked a carrier to do this once to track a fraud suspect http://www.wired.com/2013/04/verizon-rigmaiden-aircard/all/