The Fourth Amendment of the United States Constitution protects citizens of Pennsylvania from unlawful searches and seizures. Fourth Amendment protections are a foundation of criminal defense in the Keystone State and across the country. Understanding exactly what the Fourth Amendment protects, and what it does not protect, is something that all citizens should make a point of learning. For example, if a search warrant or an arrest warrant is issued, the search or seizure that the warrant authorizes is presumed legal under the Fourth Amendment.
On the other hand, for a law enforcement officer to perform a search or arrest someone without a warrant, a legally-recognized exception to the Fourth Amendment's protection must exist. One such exception - a particularly important one with which to be acquainted - is the "plain-view doctrine." Under the plain-view doctrine, law enforcement officers can seize or search contraband that is in "plain view," if several criteria are met.
First, the law officers must be in an area that is not constitutionally protected. Examples of these are public spaces, streets and highways or entering a premises pursuant to a valid warrant. Second, the object must be in "plain view," which means that the object must be visible without further inspection or searching. Objects that are visible through a car window can be considered in "plain view."
Lastly, the incriminating nature of the object must be "immediately apparent." "Immediately apparent" means that no additional inspection is needed to recognize the incriminating nature of an object. Drug paraphernalia laying on a table or a car seat, for example, is generally subject to seizure and inspection. On the other hand, if the contraband is in a container that is not incriminating in itself, such as an opaque box, it will usually not be considered to be in "plain view." It is the contraband itself that must be visible - not the container in which it's hidden.
It is also important to note than law enforcement officers must have probable cause to seize an object that is in plain view. For example, a stereo system is not incriminating in itself. However, if law enforcement officers have received a report of a stolen stereo system that matches the description of one that they observe in a vehicle they've stopped, they may have probable cause to search the vehicle under the plain-view doctrine.
Source: FindLaw.com, "Annotation 4 - Fourth Amendment," accessed Mar. 26, 2018