Cellphones do a lot more than allow us to communicate with the world: They also leave a record of where we’ve been. Because a cellphone typically connects to the nearest cell site, a record of the sites a phone has linked up with give a general indication of where the phone has been. And with a phone generally goes its user.
We know this kind of location information—sometimes called cell site location information (CSLI)—is accessible. The question is whether an officer or a prosecutor needs a warrant to access it. (A related question is whether an officer needs a warrant to track you with a "stingray.")
Courts have provided opposing answers. Some say that a court order that’s based on something akin to the lower reasonable suspicion standard is enough to entitle the government to a user’s past cellphone location data. Others have held that warrants based on probable cause are necessary.
As of 2016, though, federal courts were aligning in the position that warrants aren't necessary for CSLI. In United States v. Graham, for example, the U.S. Fourth Circuit Court of Appeals held that the government didn't violate the Fourth Amendment by getting historical CSLI from a cellphone provider without a warrant. Law enforcement obtained the data in order to approximate the defendants' locations at the times the relevant crimes happened. The court found that the defendants didn't have a reasonable expectation of privacy in the CSLI largely because, as cellphone users, they "voluntarily conveyed" it to the cell provider by making and receiving calls and texts on their phones. (United States v. Graham, No. 12-4659 (4th Cir. 2016).)
The law on access to CSLI could remain less than clear until the U.S. Supreme Court decides the issue. Even then, though, if the Court were to decide that officers need not apply for warrants, state courts could rule otherwise based on their own laws.
For more on this issue, including a 2015 federal decision, see the blog post Cellphone Location Information: Warrant Required?