When you have your phone's Wi-Fi turned on, even if you're not connected to anything, you broadcast the networks you've connected to, which in turn can reveal your location history. Ars demonstrated with a handful of test phones and a low-powered Wi-Fi monitor.
The results were not surprising to us, but they are still eye-opening, and indicative of the security and privacy risks that result from wandering around with Wi-Fi turned on but not connected. We were able to match specific devices with recent (and some not really recent) movements of the owners of the phones—where they worked, where their homes were, and in some cases where they had shopped recently—using publicly available Wi-Fi base station mapping data.
Off goes the Wi-Fi.