According to “Privacy Threats through Ultrasonic Side Channels on Mobile Devices”, by Daniel Arp, Erwin Quiring, Christian Wressnegger and Konrad Rieck, ultrasonic side channels can be used to track mobile devices: their location and the user’s activity.
An ultrasonic beacon is embedded in audio from a computer or internet-connected device (such as a Samsung TV), and this is used to track the location and activity of a user across different platforms.
These techniques are already being used. One important implication of this method is the danger it presents to air-gapping. An air-gapped computer (or mobile device–let’s call it an unmobile device) needs to be physically isolated and distant from any other device attached to the internet. The ideal situation is one in which the air-gapped computer is in a room that cannot emit RF. A spectrometer in that room will detect no RF whatsoever.
In practice, electronic devices within that room are likely to leak and emit a certain amount of RF when they are on. But those frequencies will be particular each device, and it is easy to account for such RF.