The author stumbled upon near-ultrasonic ‘pilot tones’ used in PA systems to measure health status by being present in many multi-loudspeaker systems like Zenitel VPA. Although inaudible to most humans, some may still hear them, especially if the frequency drops below 20 kHz. The tones are not believed to be harmful weapons but rather a misconfiguration. The tones are often modulated by background music and can be affected by environmental factors like people moving about. Doppler shift effects were discovered when recording around trains and in underground car parks. The author suggests using the tones for various practical purposes like speed traps, navigation, and advertising.
https://www.windytan.com/2024/06/ultrasonic-investigations-in-shopping.html