The D.C. Circuit ruled last month in United States v. Maynard that the use of a tracking device to follow a suspect over an extended period of time is a search within the meaning of the Fourth Amendment and therefore requires a warrant. The decision is founded on the idea that a person can retain a reasonable expectation even in information that he exposes to the public -- in this case, his whereabouts in public places. This rationale could thus have implications far beyond the tracking-device context, and affect how courts assess whether people retain legitimate privacy interests in other information they reveal to others -- whether their communications providers, financial institutions, or Facebook buddies. But the court's ruling conflicts with the decisions of at least three other federal circuits, and is based on a very narrow interpretation of Supreme Court precedent. We may therefore be tracking this case right up to the Supreme Court.