Stringray devices are machines that mimic cell towers and record all data within its range. It simulates the call out from a tower to all cell phones. Even if the phones are not turned on.
The problem with these things is that they tend to evolve into something insidious over time...Nevermind keeping them secret. That tends to degrade over time as well. Other governments have this technology also.
The map below tracks what we know, based on press reports and publicly available documents, about the use of stingray tracking devices by state and local police departments. Following the map is a list of the federal agencies known to have the technology. The ACLU has identified 70 agencies in 23 states and the District of Columbia that own stingrays, but because many agencies continue to shroud their purchase and use of stingrays in secrecy, this map dramatically underrepresents the actual use of stingrays by law enforcement agencies nationwide.