Here's Kim Zetter writing in Wired:
A non-disclosure agreement that police departments around the country have been signing for years with the maker of a cell-phone spy tool explicitly prohibits the law enforcement agencies from telling anyone, including other government bodies, about their use of the secretive equipment, according to one of the agreements obtained by an Arizona journalist.
The NDA includes an exception for "judicially mandated disclosures," but no mechanisms for judges to learn that the equipment was used. In at least one case in Florida, a police department revealed that it had decided not to seek a warrant to use the technology explicitly to avoid telling a judge about the equipment. It subsequently kept the information hidden from the defendant as well.
A copy of the contract was obtained from a police department in Tucson, Arizona, which signed the agreement in 2010 with the Harris Corporation, a Florida-based maker of the equipment used by the department. The police department cited the agreement as one of the reasons it withheld information from a journalist who filed a public records request seeking information about the department's use of the equipment.