www.theregister.co.uk/2014/07/03/eff_android_wifi_tracking_bug/, posted 2014 by peter in android mobile privacy wifi
Of particular concern are newer Android gadgets, specifically those running Android 3.1 "Honeycomb" or later. That version of the Google OS introduced a feature called Preferred Network Offload (PNO), which has a habit of broadcasting the names of the last 15 Wi-Fi networks a device has joined, even when the screen is off.
The idea is to conserve battery by allowing a phone to connect to known Wi-Fi networks even while in sleep mode, since Wi-Fi uses less power than the mobile data radio. The problem, the EFF says, is that your wireless network history can give a worryingly accurate and thorough picture of your movements.