Reasonable Expectation of Privacy
The Fourth Amendment to the United States Constitution guarantees that people will be safe from unreasonable searches and seizures. In the Supreme Court case, Katz v United States, the Court developed the idea of a “reasonable expectation of privacy.” The Katz case dealt with a wiretap on a public phone booth as police attempted to thwart a gambling ring. The Court stated that there was an expectation of privacy in a phone conversation, even if that conversation took place in a public phone booth.
The Court also ruled, though, that anything that a person knowingly exposes to the public, regardless of location, is not protected by the Fourth Amendment. A person must keep something private for the Fourth Amendment to apply.
To determine whether there exists a “reasonable expectation of privacy,” a two-prong test has been established.
First, the person must show a “subjective” expectation that his activities or items would be private.
Second, the individual must show that his subjective expectation of privacy is one which society considers reasonable.
To illustrate how this test works, let’s consider cases in which there is and is not a reasonable expectation of privacy.
People have reasonable expectations of privacy in their own person, house, vehicles, and business offices. They also have a reasonable expectation of privacy in their personal communications such as telephone calls, letters, and journals. Moreover, if a person needs a key or password to gain entry into a space, whether they are physical or electronic, there is also an expectation of privacy in that space. Examples here include cell phones, computers, bank safety deposit boxes, lockers, and luggage.
In contrast, people have no reasonable expectation of privacy in public spaces where their actions, possessions can be seen or conversations heard, again whether the content at issue is physical or virtual. Some examples of such places are public sidewalks, venues hosting sporting events and public parks. There is no reasonable expectation of privacy in discarded garbage left in the street. Electronically, there is also no reasonable expectation of privacy in phone numbers dialed, in the GPS location of a vehicle, electronic bank records, communications via the Internet, including e-mail, chat room or social media, in files accessible through file sharing software, in information provided by customers to Internet providers, including identifying address information.
A person also has no reasonable expectation of privacy in odors emanating from one’s person or luggage. This is why, for example, a police officer who smells alcohol on a driver’s breath may use that fact as evidence against the driver. In fact, a 2005 United States Supreme Court decision ruled that police dogs trained to sniff narcotics that indicate the presence of narcotics in a car could provide the probable cause necessary to allow police to search the vehicle.
Furthermore, a passenger in a car has no reasonable expectation of privacy in the contents of that car. So, for example, if a vehicle’s driver consents to the search of a car, materials found in the car that belong to a passenger can be used against that passenger. The driver, on the other hand, does have a reasonable expectation of privacy in the contents of the vehicle and thus can refuse police permission to search the vehicle (unless the police otherwise have a legal right to search it).
By the same token, while a homeowner (or renter) has a reasonable expectation of privacy in the contents of the home, a guest in a house has no reasonable expectation of privacy in that house. If the home owner consents to allow police to search a house, the possessions of the guests in that house are fair game.
Even in places and in items where there is generally a reasonable expectation of privacy, exceptions exist. For example, even within one’s home or property, the “open-field” doctrine provides that if something on a person’s property is easily visible to the public without the need to be physically on the property (e.g., from the air from the street) then there is no expectation of privacy. For example, the police can use photos taken by police helicopters above one’s property of items that are visible from the air.
The expectation of privacy is reduced for people who are public employees. Employees who hold safety or security sensitive jobs such as bus drivers, train conductors, law enforcement officers or laboratory workers, have reduced expectations of privacy. Because of the trust society is putting in these individuals, that compelling state interest overrides the individual’s expectation of privacy. These people may, for example, be subjected to drug testing that the government could not subject an ordinary person to. 
Moreover, when people voluntarily enter space in which the government has an elevated security interest, the expectation of privacy is likewise reduced. The most obvious example occurs at airports. Transportation Security Administration agents may subject airline passengers and their luggage to virtually unlimited searching with virtually no cause, which would never be allowed in most other contexts.
Rules relating to the doctrine of expectation of privacy do not operate within a vacuum. Reasonable expectations of reasonable people, factors including location, nature of the objects or data and many other factors must be considered. As such, whether a reasonable expectation of privacy exists is a matter that is determined on a case by case basis, and is fact-specific.
 U.S. Const. amend. IV
 Katz v United States, 389 U.S. 347, 88 S.Ct. 507, 19 L.Ed.2d 576 (1967)
 68 Am.Jur.2d Searches and Seizures §9
 15B Am.Jur.2d Computers and the Internet §28
 Illinois v. Caballes, 543 U.S. 405 (2005)
 Rakas v. Illinois, 439 U.S. 128, 99 S. Ct. 421, 58 L. Ed. 2d 387 (1978).
 Minnesota v. Carter, 525 U.S. 83, 119 S. Ct. 469, 142 L. Ed. 2d373 (1998)
 Skinner v Railway Labor Executives’ Ass’n, 489 U.S. 602, 109 S.Ct. 1402, 103 L.Ed. 685 (1989), National Treasury Employees Union v Raab, 489 U.S. 656, 109 S.Ct. 1384, 103 L.Ed. 639 (1989), National Treasury Employees Union v Yeutter, 918 F.2d 968 (D.C. Cir. 1990), National Federation of Federal Employees v Cheney, 884 F.2d 603 (D.C. Cir. 1989)