The Electronic Frontier Foundation has released an open source project called Rayhunter. It is designed to run on an inexpensive (~$20) mobile hotspot and look for signs of mobile spying devices called cell-site simulators. Also known as Stingrays or IMSI catchers, they masquerade as legitimate cellphone towers, tricking phones w/in a certain radius into connecting to the device rather than a tower.

https://www.eff.org/deeplinks/2025/03/meet-rayhunter-new-open-source-tool-eff-detect-cellular-spying

Meet Rayhunter: A New Open Source Tool from EFF to Detect Cellular Spying

Rayhunter is a new open source tool we’ve created that runs off an affordable mobile hotspot that we hope empowers everyone, regardless of technical skill, to help search out cell-site simulators (CSS) around the world.

Electronic Frontier Foundation
@briankrebs I built one. Haven't found a stingray yet, thought I know CPD and the Sheriff have some.
@Sempf @briankrebs same. Putting it in my car so I can test a greater number of places