Any time you use your phone with signal activated, even 2G signal on a dumb phone, your location can be triangulated using data on what cell towers you're pinging and how close you are to each one. This data is normally only available to outside parties down to roughly city level, but your carrier's records may hold more detailed information on where you've been, albeit nowhere near as accurate as GPS tracking. After a case involving two men suspected of robbing a string of Radio Shack stores and T-Mobile outlets around Detroit hit the courts, debate sparked as to whether this location data was constitutional to obtain without a warrant.
After the two men's appeal hit the court, it didn't take long for a decision to be rendered. The data was not protected by the constitution and could be freely requested by law enforcement, so long as they could show it was "relevant and material to an ongoing criminal investigation." This means that, even without probably cause, government elements and law enforcement can request this data, and even court order this data, at any time and regarding anybody whose location data may prove useful in an investigation, even if they're not a person of interest. A good example would be somebody whose number is given to police at the scene of an accident. If this person gives details of a major, perhaps criminal crash, the cops would be free to pull this data to ensure the person really was at or near the scene of the crash around the time it happened. Similar rulings are prevalent in many districts, which will likely lead to the adoption of this policy as law, unless the Supreme Court has to get involved.
Essentially, the courts ruled that the location data based on cell tower triangulation amounts to business records for the carrier, meaning it isn't personal data owned by the person who created it. Rather, the location data, being a business record, is not protected by the constitution. As for the two robbers who kicked off the whole debate, they were eventually found guilty and sentenced.