Pre-6.0 Android permissions aren't granular and are 'take it or leave it' approved at first install. Some apps install services that run at startup. This would make for a rather excellent monetizatation/interactivity strategy for a chat app, which already has all the necessary permissions for 'normal' operation (think Snapchat LIVE or Moments).

Sonic networking in action: [1] http://smus.com/ultrasonic-networking/ [2] https://gigaom.com/2014/06/26/chromecast-will-use-ultrasonic...