But most carriers default their phones to auto-connect to open WIFI to save themselves bandwidth.
I'm thinking that must be a British thing. My GS3 (on Virgin/Bell) in Canada doesn't autoconnect to anything WiFi unless you've previously explicitly connected to a given network and AFAICT, there's no option to even make it do so.
It's not a british thing, its apparently how android and maybe iphones work. Even though you were not actually connecting to the wifi access point as it came into range your phone does a little hello to get a signal back and determine range and stuff that included its MAC address. They then logged the MAC and monitored whether the signal was getting stronger or weaker in order to figure out your rough direction of movement. Apparently all this was done even when you just walked past the bloody things even though you never actually associated the device with the access point.
I have been walking past these things for months so they must have a nice track showing my walk to work every day as I am fairly sure I walk past a few of them between Bank and Shoreditch.