A researcher developed an affordable, distributed mobile tracking network that could take advantage of weaknesses in the way mobile devices probe for Wi-Fi signals to keep tabs on users’ movements and intercept data from their devices. The idea was to create a “dumb drone” out of the device so that it would take data collected from victim devices and push it to a central server using a VPN. For example, by placing a number of drones in popular London Underground stations, Cuthbert was able to track the whereabouts of users as they passed through the stations.”]
Source: https://www.darkreading.com/attacks-breaches/malicious-mobile-tracking-made-easy