On Fitbit activity-tracker devices, certain addresses never change. According to the popets-2019-0036.pdf document, this leads to "permanent trackability" and "considerable privacy concerns" without a user-accessible anonymization feature. The devices, such as Charge 2, transmit Bluetooth Low Energy (BLE) advertising packets with a TxAdd flag indicating random addresses, but the addresses remain constant. If devices come within BLE range at one or more locations where an adversary has set up passive sniffing, the adversary can determine whether the same device has entered one of these locations.