It seems pretty likely that a number of foreign governments are running cell phone interceptor towers in DC. Were our government on the ball, I would expect these towers to be taken down quickly and consequences issued either in diplomatic expulsions, EMP shenanigans, or some other nasty deterrent that would ensure the safety of our government's communications from prying foreign governments. This apparently has not happened, with the towers being discovered by a private company touting an expensive anti-interception phone technology who reported it to the FCC and let the rest of us know via press release.
An individual firm or even the spy chasers have a limited number of people to run around and seek out these interceptor towers. But making a phone app that identifies local towers and alarms on the appearance of new ones shouldn't be that hard or expensive. Widely release that app and you have a way to pick up the spies no matter where they are. Applications like Llama provide a functional model for finding signal and AntennaSearch provides a database to filter out existing towers.
This is a militia activity that is in response to a real threat actually being deployed against the US. You don't have to run around in camo or make any large sacrifice, just run an app occasionally on the smartphone you already own. It will make it hazardous for foreign enemies to deploy the technology against us. It will also catch domestic enemies deploying the technology in violation of the law. Legitimate use by law enforcement will have paperwork filed and the reports of this software will file will hit that paperwork and not raise alarms.