Audio beacons aren't plausible to me as a mobile app developer. Mobile OSes have been tightening their privacy controls for quite some time. At this point you can't run an Android app in the background without the user knowing. You have to explicitly request access to the microphone. In recent Android and iOS versions, the user will be notified about which apps used the microphone when. Besides, constantly recording and analyzing an audio stream would have a noticeable effect on battery life.