“Most devices send ‘probe requests’ akin to a town crier shouting out the names of networks which the device has previously connected to, so that a nearby base-station that matches any of these requests can respond. Place several Wi-Fi base-stations in a shop, then, and you can pick up these probe requests, extract the device IDs, trilaterate the positions of the devices sending them, and thus track the movements of individual shoppers, seeing which racks or displays they stop at, and what paths they follow through the store. This is arguably just the latest development in the well-established field of ‘retail science’. This was once done using video cameras.”
http://www.economist.com/blogs/economist-explains/2013/07/economist-explains-13
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