As our nation around the world struggle with the threat of terrorist attacks and violence from both foreign and domestic sources, we will be forced to engage with the ever-present tension between security and civil liberties. In a 2001 article in the Atlantic, esteemed jurist Richard Posner said –
[Civil libertarians] treat our existing civil liberties—freedom of the press, protections of privacy and of the rights of criminal suspects, and the rest—as sacrosanct, insisting that the battle against international terrorism accommodate itself to them. I consider this a profoundly mistaken approach to the question of balancing liberty and security. The basic mistake is the prioritizing of liberty.
Whether one agrees with Judge Posner’s balance of security and liberty or takes a stance more akin to the ACLU’s goal of keeping America “safe AND free,” it is important that we remain engaged in this debate. For civil liberty minded individuals, it is important to shine light on how this balancing can erode civil liberties overtime, even if the erosion may be justifiable in some cases. Further, it is important to recognize that threats to civil liberties come not just from security-focused laws, but also from new security technologies.
Late last month, at the Macy’s Thanksgiving Parade, the NYPD used a new technology that allowed them to more easily detect explosives in large moving crowds. Termed “vapor wake” dogs, these K9 cops are able to detect and track the scent of explosive devices off of moving targets. This new “technology” has the potential to combat evolving security threats that are increasingly difficult to detect when aimed at crowded. As a recent article in Rolling Stone pointed out, these dogs are also efficient. They can check concert or parade-goers as they pass by, instead of having to stop and check the individuals, which can create bottleneck at attraction entrances.
The advanced training these dogs receive has the potential to make security searches less intrusive, public gatherings safer, and individuals more secure. This advancement, however, also has the potential to subject individuals to a possibly unconstitutional search, each time they pass through a public space.
Courts have not yet addressed the constitutionality this “technology.” Further, based on current Fourth Amendment precedent, it is unclear whether courts will even consider these sniffs and subsequent tracking a search. In US v. Reyes, for example, the Fifth Circuit found that a dog alerting law enforcement to the presence of drugs on a person walking within four to five feet of the dog was not a search under the Fourth Amendment. This non-contact sniff was not deemed to interfere with the defendant’s “justifiable… reasonable, or… legitimate expectation of privacy.” But the Ninth and Fifth Circuits have determined that “up-close” dog sniffs do constitute a search.
Whether this kind of search would be considered unreasonable under the Fourth Amendment is another question. The Supreme Court has held that a warrantless search is per se unreasonable, subject to a number of exceptions. Exactly what these exceptions encompass, however, is less clear. While “consent, plain view, stop and frisk, emergency, search incident to arrest, and the automobile” are the often consider the major categorical exceptions, exactly what falls under each category is usually based on a balancing test between the individual’s privacy interests and government’s interest in the search.
The present use of the “vapor wake” dogs would likely pass muster, as it would be hard to argue that one’s privacy interests in a public space outweigh the government’s interest in preventing explosive devices in crowded areas. The training and use of these dogs, however, will probably only further evolve. On which side of the reasonable search line future uses of these dogs, for instance to detect other illicit substances like drugs, will fall cannot be determined based on current precedent. It is important that individuals remain mindful of the development of such technology and how the civil liberties law surrounding it will govern future uses.
 U.S. v. Reyes, 349 F.3d 219, 223 (5th Cir.1982)
 B.C. v. Plumas Unified School District, 192 F.3d 1260, 1266 (9th Cir.1999).
 See Horton v. Goose Creek Independent School District, 690 F.2d 470 (5th Cir.1982); U.S. v. Kelly, 302 F.3d 291 (5th Cir. 2002).
 Coolidge v. New Hampshire, 91 S.Ct. 2022, 2032 (1971).
 See Comment, Search and Seizure: From Carroll to Ross, The Odyssey of the Automobile Exception, 32 Cath. U. L. Rev. 221, 221-22 (1982). The emergency” category is often also referred to as exigent circumstances. See, e.g. Birchfield v. North Dakota, 136 S. Ct. 2160, 2173 (2016).
 Riley v. California, 134 S.Ct. 2473, 2484 (2014).