Today’s unanimous Supreme Court ruling in United States v. Jones makes it clear that government installation and use of GPS tracking devices is a Fourth Amendment “search”—but it may be the concurring opinions, rather than Justice Scalia’s majority opinion, that are most significant for Americans’ privacy in the 21st century.

As Jim Harper notes, Justice Scalia ruled on the relatively narrow grounds that installing the tracking device involved physical intrusion on the suspect’s property, triggering Fourth Amendment protections.  Yet as Justices Alito and Sotomayor observe in separate concurrences—and as I pointed out in a previous post on this case—there are plenty of means for tracking a target’s location in public that don’t require such intrusion. One of the most popular with law enforcement is cell-phone tracking, either by means of a court order demanding records from the phone company directly, or through the use of devices known as “Stingrays” or “Triggerfish.” There’s also the use of license-plate recognition cameras, and even aerial surveillance drones.  The broader question that’s crucial to determining the extent of our privacy rights in the long term, then, is the one Scalia’s opinion pointedly declines to reach: Does prolonged, technologically-assisted location surveillance impinge on a citizen’s “reasonable expectation of privacy,” even when it does not require physical intrusion?

Justice Alito, joined by three other justices, says that it can indeed—and in this case, did. The placement of a tiny device on the undercarriage of a car parked in a public place, Alito argues, does not sufficiently “interfere” with a suspect’s property interests to constitute a Fourth Amendment “seizure,” nor is it a “search” until police activate and begin monitoring the device. If the police had simply slipped a business card into the tire, after all, the physical intrusion would be too minor in itself to count as an actionable trespass. Instead, Alito insists, it is necessary to proceed to the harder question of whether such intensive location monitoring violates our reasonable social expectations of privacy, even as we move around in public. Though the concurrence is reluctant to say exactly when that expectation is breached, Alito notes that round-the-clock surveillance over a full month would be so costly to carry out by conventional physical observation that it exceeds what reasonable people expect—and so triggers the Fourth Amendment’s warrant requirement.

Perhaps most intriguing is Sotomayor’s brief concurrence. For Sotomayor, either the property rationale relied on by Scalia or the “expectations” analysis deployed by Alito would suffice to find a Fourth Amendment violation here. That’s crucial, because it means that there are at least five votes on the current Court for the view that we have some Fourth Amendment protection against intensive, high-tech location tracking, even in public, and even when the method doesn’t require physical intrusion. Yet even more important than that may be this passage:

More fundamentally, it may be necessary to reconsider the premise that an individual has no reasonable expectation of privacy in information voluntarily disclosed to third parties. […] This approach is ill suited to the digital age, in which people reveal a greatdeal of information about themselves to third parties in the course of carrying out mundane tasks. People disclose the phone numbers that they dial or text to their cellular providers; the URLs that they visit and the e-mail addresses with which they correspond to their Internet service providers; and the books, groceries, and medications they purchase to online retailers. […] But whatever the societal expectations, they can attain constitutionally protectedstatus only if our Fourth Amendment jurisprudence ceases to treat secrecy as a prerequisite for privacy. I would not assume that all information voluntarily disclosed to some member of the public for a limited purpose is, for that reason alone, disentitled to Fourth Amendment protection.

This is a pretty big deal. Fourth Amendment scholars have been warning for decades—and with increasing alarm—that modern communications technology could turn constitutional privacy protections into an empty formality if we’re regarded as waiving those protections whenever we “expose” information to a third party. It is inherent to the nature of the Internet and mobile telecommunications, after all, that almost everything we do online—and, increasingly, much that we do offline as well—leaves a trace in the vast databases of one corporation or another.

Sotomayor’s concurrence signals a recognition that we need to move beyond what privacy scholar Daniel Solove has called “The Secrecy Paradigm,” which assumes that whatever is not totally secret (or very nearly so) is effectively “public.” In other words, if your Internet provider has a record of every Web site you visit, there’s no invasion of privacy when the government decides to have a look at the list. At least one Justice, evidently, recognizes that this is an indefensible inference—and one hopes she’s not alone.