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It's not just iPhones, it's any wireless device that is actively scanning for a previously associated access point.

These days iPhones will change the MAC address while doing wifi scans to a random one so you can't track individual iPhones anymore.

Some Android phones allow you to toggle this feature though it wrecks with MAC filtering at home and elsewhere if required.

Usually devices will randomize their MAC when scanning, and pick a MAC associated with the SSID: https://android.stackexchange.com/questions/225839/android-1...