Police have been using the metadata gleaned from cellphone tower interactions for years. And they have done it without a warrant. This issue, whether cellphone location data obtained from the telecoms requires a warrant had fallen through the cracks of Fourth Amendment law.
Because it wasn’t an old fashioned phone tap, law enforcement attempted to treat it as if it was not a "search" and therefore outside the Fourth Amendment’s warrant requirement.
A recent U.S. Supreme Court decision ruled that a GPS tracker attached by law enforcement to a vehicle was a search, but did not reach the issue of whether it was a search that required a warrant.
In a case from the Supreme Court of Florida last month, the Court ruled that use of location information obtained from cell towers of the telecom was a search for Fourth Amendment purposes and the police need a warrant to use that information to track a suspect, who in this case was an alleged drug dealer.
Phone taps require a warrant based on probable cause because they disclose the content of a private communication. But phone numbers and location data are necessarily disclosed to your telephone company when you dial a phone number and when you move about with a cellphone, as the phone automatically switches between cell towers.
Because this information is voluntarily disclosed to a third-party, the telecom company, the U.S. Supreme Court held in a 35 year old case, that you had no reasonable expectation of privacy in that data, and permitted pen register or a "trap and trace," which only record the incoming and outgoing phone numbers, to be used without a warrant.
Wired.com, "Cops Need a Warrant to Grab Your Cell Tower Data, Florida Court Rules," Kim Zetter, October 17, 2104