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 Cap…, loc. 430