Wearable technology can make invisible things visible. If someone is trying to use ultrasound beacons to track people where you are, you could be alerted via wearables (lights, sounds, vibration). LEDs would be good as you could get various information about the beacons at a glance.
A cell phone can detect all kinds of things like this. Tiny RFduino attached to wearable can receive data from phone/laptop, and represent visually (etc). Could also alert user to RF signals/wifi, bluetooth/NFC beacons, any imaginable data from geo APIs, etc.
This proof-of-concept uses an Android phone to detect ultrasound that might indicate a tracking beacon. It sends BLE signals to a "wearable" device with two indicator lights. One of the lights indicates that there is generally a high amount of ultrasound. A second light indicates that a specific signal was recieved on two specific preprogrammed frequencies.
More / Future Ideas
More analysis of real tracking protocols like Lisnr, Chirp,io etc. would provide the ability to more accurately identify real beacons or even analyze their content.
Add support for BLE, NFC beacons.
Alex Glo suggested that API on crime data could alert you via wearable if you're entering for example an area with higher crime.
Matt Bellis pointed out that if we wanted to, we could detect ultrasound signals using simple analog circuits which would be much smaller and use a lot less power than doing fast Fourier transforms on digital data - so that ultrasound beacon detectors could be made very small and low power - built into the wearable instead of connected to another device over Bluetooth Low Energy. But now I don't really know how to do that except in theory, so I'm using software!
Inspiration and Credits
Using Ultrasonic Beacons to Track Users https://www.schneier.com/blog/archives/2017/05/using_ultrasoni.html
Sample Android App for RFduino (used some code Copyright 2013 Lann under MIT license) https://github.com/lann/RFDuinoTest