This reminded me of „Ultrasonic investigations in shopping centers”[1]

> Ultrasonic pilot tones, commonly used in PA systems to monitor speaker connections, are often modulated by background noise or movement. These tones can be used for various purposes, such as speed detection, escalator monitoring, positioning codes, ad delivery, and finding quiet areas in shopping centers.

[1] https://www.windytan.com/2024/06/ultrasonic-investigations-i...