What got my eyes was the following in the press:

That's freaky. The worst thing is that having a cross tracking device can also help you in being tracked even though you're using anonymizing tools:

Before going into the details of the projects, I've been doing some research to understand what was at stake.

• Startup uses ultrasound chirps to covertly link and track all your devices
• Ultrasound Tracking Beacons Making Things Sort of Creepy For Consumers
• How to Block the Ultrasonic Signals You Didn’t Know Were Tracking You
• Beware of ads that use inaudible sound to link your phone, TV, tablet, and PC Privacy advocates warn feds about surreptitious cross-device tracking.

Some companies and framework using those are:

• Silverpush
• Shopkick
• lisnr
• Signal36

PS: for a bonus.. the future of advertising thanks to ultrasound tracking =)