Inaudible ultrasonic tones emitted by retail speakers can track phones indoors.

Evidence

For example, the ad tracker FidZup developed “communication between a sonic emitter and a mobile phone.…” It can detect the presence of mobile phones and therefore their owners by diffusing a tone, inaudible to the human ear, inside a building: “Users installing ‘Bottin Gourmand,’ a guide to restaurants and hotels in France, would thus have their physical location tracked via retail outlet speakers as they move around Paris. — Shoshana Zuboff, The Age of Surveillance Capitalism, 430

Categorization

• SHARE: Send to anyone in retail, privacy, or security tech — the inaudible-speaker-tracking technique is exactly the kind of specific, tech-narrow fact they’d want.
• CANDY: A genuinely surprising, low-context fact: businesses can track you via sounds you literally cannot hear.
• XREF: Prefigures Zuboff’s later chapters on ambient, sensor-based surveillance and connects to concrete tracking tech like Silverpush’s ultrasonic beacons from other privacy literature.