This morning, the Supreme Court agreed to hear an appeal next term of United States v. Jones (formerly United States v. Maynard), a case in which the D.C. Circuit Court of Appeals suppressed evidence of a criminal defendant’s travels around town, which the police collected using a tracking device they attached to his car. For more background on the case, consult the original opinion and Orin Kerr’s previous discussions about the case.

No matter what the Court says or holds, this case will probably prove to be a landmark. Watch it closely.

(1) Even if the Court says nothing else, it will face the constitutionally of the use by police of tracking beepers to follow criminal suspects. In a pair of cases from the mid-1980’s, the Court held that the police did not need a warrant to use a tracking beeper to follow a car around on public, city streets (Knotts) but did need a warrant to follow a beeper that was moved indoors (Karo) because it “reveal[ed] a critical fact about the interior of the premises.” By direct application of these cases, the warrantless tracking in Jones seems constitutional, because it was restricted to movement on public, city streets.

Not so fast, said the D.C. Circuit. In Jones, the police tracked the vehicle 24 hours a day for four weeks. Citing the “mosaic theory often invoked by the Government in cases involving national security information,” the Court held that the whole can sometimes be more than the parts. Tracking a car continuously for a month is constitutionally different in kind not just degree from tracking a car along a single trip. This is a new approach to the Fourth Amendment, one arguably at odds with opinions from other Courts of Appeal.

(2) This case gives the Court the opportunity to speak generally about the Fourth Amendment and location privacy. Depending on what it says, it may provide hints for lower courts struggling with the government’s use of cell phone location information, for example.

(3) For support of its embrace of the mosaic theory, the D.C. Circuit cited a 1989 Supreme Court case, U.S. Department of Justice v. National Reporters Committee. In this case, which involved the Freedom of Information Act (FOIA) not the Fourth Amendment, the Court allowed the FBI to refuse to release compiled “rap sheets” about organized crime suspects, even though the rap sheets were compiled mostly from “public” information obtainable from courthouse records. In agreeing that the rap sheets nevertheless fell within a “personal privacy” exemption from FOIA, the Court embraced, for the first time, the idea that the whole may be worth more than the parts. The Court noted the difference “between scattered disclosure of the bits of information contained in a rap-sheet and revelation of the rap-sheet as a whole,” and found a “vast difference between the public records that might be found after a diligent search of courthouse files, county archives, and local police stations throughout the country and a computerized summary located in a single clearinghouse of information.” (FtT readers will see the parallels to the debates on this blog about PACER and RECAP.) In summary, it found that “practical obscurity” could amount to privacy.

Practical obscurity is an idea that hasn’t gotten much traction in the Courts since National Reporters Committee. But it is an idea well-loved by many privacy scholars, including myself, for whom it helps explain their concerns about the privacy implications of data aggregation and mining of supposedly “public” data.

The Court, of course, may choose a narrow route for affirming or reversing the D.C. Circuit. But if it instead speaks broadly or categorically about the viability of practical obscurity as a legal theory, this case might set a standard that we will be debating for years to come.