A Justice Department document obtained by the ACLU of Northern California shows that federal investigators were routinely using a sophisticated cell phone tracking tool known as a "stingray," but hiding that fact from federal magistrate judges when asking for permission to do so.
Stingrays and similar devices essentially impersonate cell phone towers, allowing them to pinpoint the precise location of targeted cell phones (even inside people's homes) and intercept conversations. They also sweep up the data of innocent people who happen to be nearby. By withholding information about this technology from courts in applications for electronic surveillance orders, the federal government is essentially seeking to write its own search warrants.
Source: ACLU. Read full article. (link)