Are You Being Followed? Use a Raspberry Pi to Find Out

pcmag.com:

In the movies, a hero can always tell he's being followed because the goons tasked with following him never blend in. In real life, figuring out if someone is tailing you is much trickier, and can be a matter of life and death. At the Black Hat security conference, a speaker demonstrated a low-cost device that looks for the tell-tale wireless signature of bad guys on your tail.

Matt Edmondson, who works with the US Department of Homeland Security, was approached by a friend from a government agency he declined to name onstage at Black Hat. This friend worked with confidential sources, and one in particular had links to a terrorist organization. Edmondson's friend was concerned that if they were followed after meeting with the confidential source, his friend's government connections could be discovered and the source put in danger. 

The traditional spycraft method of surveillance detection, Edmondson explained, is to change your route and see who does the same—such as exiting the highway and then getting back on again. "It's really obvious the [Washington, D.C.] Beltway was designed as a surveillance-detection route," quipped Edmondson, perhaps joking, perhaps not. 

Edmondson said his friend asked if he could revisit an idea he had discussed years ago: Using network-detection technology to scan for devices that were following you. 

Even if you're being tailed by a nation-state-backed surveillance team, "isn't there still a really good chance they have a phone in their pocket?" asked Edmondson.

This works because so many of our devices are constantly trying to communicate with other devices and various wireless networks. Many mobile devices, for example, are constantly seeking familiar wireless networks to connect to.