What is Plain View Doctrine?

Legal Definition
The plain view doctrine allows an officer to seize, without a warrant, evidence and contraband that are found in plain view during a lawful observation. The doctrine is also regularly used by TSA Federal Government Officers while screening persons and property at U.S. airports.

For the plain view doctrine to apply for discoveries, the three-prong Horton test requires:

  1. the officer to be lawfully present at the place where the evidence can be plainly viewed,
  2. the officer to have a lawful right of access to the object, and
  3. the incriminating character of the object to be “immediately apparent.”

In order for the officer to seize the item, the officer must have probable cause to believe the item is evidence of a crime or is contraband. The police may not move objects to get a better view. In Arizona v. Hicks, 480 U.S. 321 (1987), the officer was found to have acted unlawfully. While investigating a shooting, the officer moved, without probable cause, stereo equipment to record the serial numbers. The plain view doctrine has also been expanded to include the sub doctrines of plain feel, plain smell, and plain hearing.

In Horton v. California 496 U.S. 128 (1990), the court eliminated the requirement that the discovery of evidence in plain view be inadvertent, which had led to difficulties in defining "inadvertent discovery."
-- Wikipedia
Legal Definition
The plain view doctrine allows a police officer to seize objects not described in a warrant when executing a lawful search or seizure if he observes the object in plain view and has probable cause to believe that it is connected with criminal activities. The incriminating character of the object should be immediately identifiable. Evidence in a container can not possibly be identified by plain observation or touch and therefore should not be seized unless it is listed in the warrant.

Analogously, pat-down searches when an officer by plain feel of the outside of clothes reasonably believes an item in the person’s pocket is a contraband or evidence.