A "stingray" is a device the size of a suitcase that impersonates a cellphone tower. It tricks cellphones into connecting with it and in that way identifies the phones' location. These cell-site simulators can penetrate walls. When they connect to a phone, they connect to all phones in the area. And when they connect, they're capable of grabbing data beyond location information, like emails and texts.
At least some law enforcement agencies use stingrays to investigate crimes—this we know. But do they obtain warrants beforehand? (The U.S. Supreme Court has established that the police typically need a warrant to see where you've been through historical cell-site location information.)
A 2015 USA Today article and other reports indicate that at least some agencies don't get warrants before using stingrays. In some states, though, they're required to. When it comes to the federal government, the Department of Justice (DOJ) generally requires that agencies under its watch get warrants before using stingrays. So, if the FBI wants to use a cell-site simulator in a routine domestic probe, its agents first have to establish probable cause to a judge's satisfaction. (But the exceptions to the DOJ policy are hardly narrow.) The Department of Homeland Security has a similar stingray policy.
The law on stingray use is quickly evolving. As courts and legislatures catch up to the technology, more cases and statutes on cell-site simulation will probably emerge.