Street-level surveillance technologies such as automated license plate readers, facial recognition systems, and cell-site simulators (CSS) have become increasingly prevalent. Among these, CSS, also known as Stingrays or IMSI catchers, are particularly concerning due to their ability to masquerade as legitimate cellular towers, intercepting and logging sensitive information from mobile devices. As such, the Electronic Frontier Foundation (EFF) has developed an open-source tool called Rayhunter to help detect and understand the use of these devices.
Source: EFF launches open-source tool that can detect unauthorized cellular spying