Could be ultrasonic tracking beacons, see here: https://www.wired.com/2017/05/hundreds-apps-can-listen-beacons-cant-hear/ https://www.wired.com/2017/05/hundred

Could be ultrasonic tracking beacons, see here: https://www.wired.com/2017/05/hundreds-apps-can-listen-beacons-cant-hear/ https://www.wired.com/2017/05/hundreds-apps-can-listen-beaco...