https://www.reddit.com/r/androidapps/comments/3tujti/ultrasound_spying_apps_beware_of_ads_that_use/
Ultrasound spying apps: "Beware of ads that use inaudible sound to link your phone, TV, tablet, and PC – "While the sound can't be heard by the human ear, nearby tablets and smartphones can detect it. When they do, browser cookies can now pair a single user to multiple devices"