How many police departments hide Stingray use from judges?

Milwaukee PD have been using a Stingray, a device that captures cell phone data by mimicking a cell tower, in secret in over 500 investigations.  They, like every other police department that receives a Stingray, first had to sign a nondisclosure agreement with the FBI, who is the agency handing out these devices.  Now Congress has some questions for the FBI.