Cell-site location data has become a hot commodity for law enforcement, and an increasingly pressing matter for courts. After the Supreme Court put the kibosh on the use of physical tracking devices without a search warrant ‒ at least for tracking over long periods ‒ in U.S. v. Jones, law enforcement is having to rely more on location data acquired from mobile carriers to track suspects. Courts remain split on whether a warrant is required to obtain cell-site location data. Adding to the split, the Fifth Circuit recently held, in In Re: Application of the United States of America for Historical Cell Site Data, No. 11-20884, that the government does not need a warrant, since a cell phone’s location information is voluntarily conveyed by the user to a third party, the mobile service provider. The “third-party doctrine” affects far more than cell-site location data, and is relevant to financial institutions, communications companies, and Internet firms that possess customer data that investigators may want to get their hands on. As we previously reported, that doctrine has been called into question explicitly or implicitly by five of the Justices in Jones. So every development regarding that doctrine could be significant for companies that maintain customer data.