Location data from your phone may fall under the protection of the 4th Amendment to the Constitution of the United States of America, and advocates from various circles, including the tech world, are making the argument that this should be the case. The conversation was started by a court case known as Carter v. the United States, wherein the court is seeking the right to obtain rough location data to track the defendant over the course of 127 days. Carter is being represented by the American Civil Liberties Union. The movement includes representatives from the Electronic Frontier Foundation, National Association of Criminal Defense Lawyers, Verizon, and a panel of experts from around the tech sphere. The base argument is that obtaining data constitutes seizure, while interpreting the data constitutes search, two activities that are restricted by the Fourth Amendment. The Fourth Amendment protects from "unreasonable" examples of those activities, and establishes the requirement for law enforcement agencies to obtain a warrant before performing most types of search and seizure procedures.
The type of location data that's presently at the center of the conversation is the somewhat less precise location data that can be gleaned from any device connected to a cellular network, with or without the involvement of GPS. This data includes a triangulation of your current location from nearby cell towers, as well as the locations of nearby Bluetooth devices and Wi-Fi networks, if available. This data tends to be less precise than GPS data, with an average accuracy of a couple dozen to a couple hundred meters, depending on network conditions. Thanks to the deployment of a larger amount of towers and small cells and more sophisticated network equipment, as well as a larger amount of mobile, IoT, and other electronic devices around at any given time, this location data has been less prone to gross inaccuracy in recent years.
The location data in question has, in the past, been considered imprecise enough to not warrant it being categorized as personal or private data. Police have used such data on a fairly routine basis for more rough usages, such as obtaining evidence of an alibi or a lack of one, putting multiple defendants near the scene of a crime at the same time, and doing other investigative tasks. Having such data require a warrant going forward could make investigations costlier and slower, which in turn means that the privacy and security advocates trying to push for this change will have an uphill battle ahead of them.