Orin Kerr—easily one of our most lucid thinkers when it comes to applying the Fourth Amendment to new technologies—argues at Volokh Conspiracy that, while it's a hard call whether the installation of a GPS tracking device to a vehicle counts as a Fourth Amendment "search" or "seizure," the Supreme Court should not treat the use of such devices as a search when it decides United States v. Antoine Jones later this term. Rather, he argues, the Court should hew to a bright line test that makes monitoring "inside" protected spaces a "search" requiring a warrant, while "outside" monitoring—as, for example, of a car traveling on public roads—is always permitted, regardless of the technological means by which that monitoring is carried out, or how extensive that monitoring is in scope or duration. This is in line with Kerr's reasoning in a thoughtful and important article about the application of the Fourth Amendment to the Internet, which I've already written about in this space.
First, it's worth noting that Kerr's core assertion—that the inside/outside distinction is already the one consistently applied by the Court—is at odds with some fairly unambiguous assertions in the very opinions he cites. Kerr argues:
You get the same results whether you get these results under the “protected areas” test that preceded the 1967 Katz case, or the Katz “reasonable expectation of privacy” test that the Court has adopted since then. The results are the same: A search occurs when the government intrudes upon a private person, house, paper, or effect, but does not occur when the government merely observes something in a public space or in a space where the government is otherwise entitled to be.
But the majority in Katz goes out of its way to deny this:
[T]he Fourth Amendment protects people, not places. What a person knowingly exposes to the public, even in his own home or office, is not a subject of Fourth Amendment protection.... But what he seeks to preserve as private, even in an area accessible to the public, may be constitutionally protected. [emphasis added]
It is true, of course, that in general our reasonable expectations of privacy will track the line between public (outside) and private (inside) spaces. But this is, crucially, a defeasible presumption, as Kerr plainly concedes when he notes that a search does not occur "when the government merely observes something in a public space or in a space where the government is otherwise entitled to be." If you have freely invited a police officer or informant into your home, where they smell marijuana, you cannot claim that any reasonable expectation of privacy has been violated, even though the odor may only be detectable from "inside" this private space. But then, as the majority in Katz suggests, the presumption should be equally defeasible on the other side: There may be cases where what can be observed in public is nevertheless reasonably expected to be private.
Indeed, Katz itself may be just such a case: It involved a bug placed on the outside of a phone booth, picking up only the faint vibrations from the conversation inside that reached the outside of the booth (though, of course, they would not have been audible to an unaided human ear that was not rather conspicuously pressed up against the glass). We might imagine, instead, that the surveillance had been of the sort shown in the opening scene of Francis Ford Coppola's masterpiece The Conversation, where a series of high-powered boom microphones and sophisticated audio filtering technology are used to capture a whispered exchange between a couple who are walking through a noisy plaza—and constantly checking to ensure that nobody is near enough to overhear. The majority supports its reasoning with the observation that Katz had stepped into a phone booth and "closed the door," but nothing about the logic of Katz depends centrally on the bugged conversation occurring "inside" that closed booth. There's no reason to think the decision would or should have been different if it had involved a scenario like Coppola's, or a cell phone call placed from a desolate beach.
Kerr does consider some situations where monitoring from a public vantage point has been regarded as a search. On Kerr's reading of the ruling in Kyllo, the use of a thermal imaging scanner to observe a house from outside, on a public street was nevertheless a search of the inside of the home. Similarly, the protection afforded to the "curtilage" of the home involves, in effect, expanding the penumbra of protection around the home so that some property literally "outside" is nevertheless "treated" as being "inside." At this point, it seems to me, the "inside/outside" distinction ceases to be a useful guiding principle and is exposed as a mere tautology. If some kinds of monitoring are "treated as inside surveillance" even though, literally speaking, they involve only the "outside" of a protected space, "inside" has just become a term of art meaning "protected."
This is even more obvious when we turn to monitoring of Internet surveillance, where Kerr hopes the "inside/outside" distinction may be usefully employed. Because while one may speak (for instance) of e-mail header information or metadata as "outside," by analogy with a mailed envelope, by contrast with the "inside" message contents, this is pure metaphor. In a literal sense, they're all just bits. Moreover, Internet communications operate on the Open Systems Interconnection model, which divides each Internet packet not into "inside" and "outside," but rather seven nested layers. Kerr allows that it is sometimes a "tricky call" where the line between inside and outside surveillance falls. But as thinking about Internet communications should make clear, this is a misleading description. There is no "fact" about the inside/outside boundary for the Court to discover as it contemplates Internet surveillance, no independent dividing line that the Court might use to determine which aspects of the communication deserve protection. Rather, the decision about how the physical metaphor ought to apply to a digital communication just is the decision about what should be protected. In considering which parts of the bitstream are "like" the envelope exterior, and which are "like" the letter within—or for that matter, considering whether the right metaphor is a sealed letter or a postcard—the Court would have to appeal to other considerations.
Let's return to the GPS tracking case, applying some of the reasoning from a decision that might seem to support Kerr's "inside/outside" distinction: California v. Ciraolo. Here, the Court held that aerial observation by police of a marijuana garden in the curtilage of the home was not a Fourth Amendment search. Yet their argument for this doesn't turn on a simple inside/outside binary. Rather, the Court noted:
The observations ... in this case took place within public navigable airspace ... in a physically nonintrusive manner; from this point, they were able to observe plants readily discernible to the naked eye as marijuana .... Any member of the public flying in this airspace who glanced down could have seen everything that these officers observed..... In an age where private and commercial flight in the public airways is routine, it is unreasonable for respondent to expect that his marijuana plants were constitutionally protected from being observed with the naked eye from an altitude of 1,000 feet.
In this case, monitoring of the curtilage is clearly treated as "outside" surveillance given the method employed—a method that, crucially, is "routine" for members of the public who are not police officers. As the Court makes clear in a footnote, such aerial surveillance might become intrusive enough to qualify as a search if it involved "technology which discloses to the senses those intimate associations, objects or activities otherwise imperceptible to police or fellow citizens."
Let me posit that almost all of us normally expect privacy for communications and activities that, though they occur in a public space, would be "imperceptible... to fellow citizens" whose senses were not augmented by sophisticated technology, and we routinely rely on that expectation in practice. If I am at a crowded party and wish to pull a friend aside for a private chat, I may seek out an empty room and close the door behind us... but I may equally reasonably step outside, like the couple in The Conversation, to talk in hushed tones in the deserted driveway. And my expectation that this conversation is private may be reasonable even if, of course, someone could overhear us by approaching close enough to pick up our whispers with an unaided ear—at which point we would stop discussing private matters. Indeed, for people of severely limited economic means—people who may lack regular access to "inside" spaces other than thin-walled tenement apartments or even crowded homeless shelters—the de facto "privacy" of isolation in nominally "public" spaces may be the only sort realistically available.
Parallel reasoning applies if we consider movements rather than conversations. A person driving down a public street is subject to observation—but many contextual features determine whether they reasonably expect such observation in a specific case. Someone who undertakes a long journey along a desolate highway at night will, normally, reasonably believe that they are not being followed or observed if they don't see headlights trailing them. Two people who rendezvous at a cabin deep in the woods, arriving separately, will not be less shocked that their liaison is being monitored because the dirt road to their meeting point is "public."
A simple, binary "inside/outside" bright line rule for deciding how the Fourth Amendment applies to high-tech surveillance is appealing because of its seeming clarity and straightforwardness. But clarity is not a virtue if it is achieved at the price of the underlying interests the Fourth Amendment protects. After all, "no warrants are ever required" would be the clearest, simplest rule of all.