234 Android apps have been identified as using your phone to monitor TV advertising effectiveness:
The apps silently listen for ultrasonic sounds that marketers use as high-tech beacons to indicate when a phone user is viewing a TV commercial or other type of targeted audio. A representative sample of just five of the 234 apps have been downloaded from 2.25 million to 11.1 million times, according to the researchers, citing official Google Play figures. None of them discloses the tracking capabilities in their privacy policies.
Source: More Android phones than ever are covertly listening for inaudible sounds in ads | Ars Technica
