In U.S. v. Jones, decided in January, the Supreme Court ruled that police need a warrant to track a vehicle by attaching a GPS device to it. But because the decision hinged on the physical intrusion involved in attaching the device, it left unclear what limits the Fourth Amendment imposes on tracking that does not require police to touch the target's property. Now the American Civil Liberties Union, based on documents collected from about 200 law enforcement agencies, reports that police often collect cellphone location data without a court order. While some police departments obtain such data without a warrant only in life-threatening emergencies, some do so routinely, while others let cellphone carriers decide how much legal authorization is necessary. "Law enforcement agencies' tracking policies are in a state of chaos," the ACLU says, "with different towns following different rules —or in some cases, having no rules at all."

Based on the documents obtained by the ACLU, The New York Times reports that police departments in Nevada, North Carolina, and other states "have gotten wireless carriers to track cellphone signals back to cell towers as part of nonemergency investigations to identify all the callers using a particular tower." Thanks to modern technology, a California training manual explains, "subtler and more far-reaching means of invading privacy have become available to the government." According to the Times, a Nevada training manual noted that cellphone tracking "had been 'misused' in some standard investigations to collect information the police did not have the authority to collect." It warned that "continued misuse by law enforcement agencies will undoubtedly backfire." In fact, the Times says, "Many departments try to keep cell tracking secret...because of possible backlash from the public and legal problems." Speaking of which, Reason Contributing Editor Julian Sanchez, a privacy specialist at the Cato Institute, argues that collection of cellphone location data is the controversial surveillance practice that the Obama administration is conducting under its secret interpretation of Section 215 of the PATRIOT Act.

If the legal authority for such warrantless surveillance were clear, law enforcement agencies would not be so shy about admitting that they do not bother to obtain judicial authorization. The Supreme Court has ruled that information about you held by third parties is not covered by the Fourth Amendment (although access to it may be restricted by statute) because you have voluntarily relinquished it. The information about where you are (or were) that cellphone companies have arguably falls into this category. Then again, the logic of Justice Samuel Alito's concurring opinion in U.S. v. Jones—which said collecting information about a person's whereabouts, even in public places, can reveal so much that it constitutes a "search" under the Fourth Amendment—suggests a warrant is required for at least some kinds of cellphone tracking. A better approach, as suggested by Justice Sonia Sotomayor in her concurring opinion, would be to rethink the notion that sharing information with businesses for particular reasons means surrendering all constitutional claims to privacy concerning that information. Rather than count on the Supreme Court, the ACLU is urging statutory restrictions on cellphone tracking by the government.