Forbes reporter Thomas Fox-Brewster looks at SilverPush, which uses inaudible sound beacons to track people. The company places the sound in a TV ad that can be recognized by a mobile app with SilverPush's beacon. "Once the sound is heard, the application knows what the user is watching," Fox-Brewster writes. The tracking technology, which has riled privacy advocates, is currently used only in India, according to Forbes.