What your phone broadcasts before you unlock it

When Wi-Fi is enabled, a phone periodically announces itself looking for networks. Historically it would name the networks it remembered, which meant a passer-by could learn where you had been simply by listening. Modern phones have largely stopped naming networks in the clear and randomise the hardware address they advertise, which was a genuine improvement. The improvement is partial. Randomisation is often disabled for networks you have already saved, so the moment you come within range of a remembered hotel or café network, your phone may reveal a stable identifier. The longer you stay in one place, the more useful that identifier becomes to anyone collecting it.

Bluetooth Low Energy exists to be discoverable, which is exactly what makes it chatty. Headphones, watches, trackers, and the phone itself advertise their presence on a regular cycle. Manufacturers rotate these identifiers to limit tracking, but accessories vary widely in how well they implement it, and an older accessory paired to a new phone can undo the phone's own protections. This is worth knowing rather than worth panicking about. The exposure is local — someone has to be near you — but airports, stations, and conference venues are precisely where being near you is easy.

The effective steps are unglamorous.

Most of this exposure comes from the search for connectivity rather than from connectivity itself. A phone with a working cellular data plan has no reason to scan for networks, no reason to join an unfamiliar one, and no reason to accumulate saved entries in every city you pass through. That is a quieter default, and it is available simply by arriving with data that already works.