The Constitution and 4th Amendment protects many types of unreasonable searches and seizures, however, it does not protect against things left out on the open where anyone, specifically an officer, can see them. The Plain View Exception to the 4th amendment requirement of law enforcement obtaining a warrant to search allows officers to search a person, bag, car, or area when probable cause is reached by observing something in plain sight.
Once the item is seen in plain view, the officer may use the established probable cause to search for more illegal items and investigate other crimes further, as long as the officer is observing from a legally obtained vantage point.
For instance, the typical plain view doctrine case involves a traffic stop for speeding or failing to stop at a stopsign, followed by the officer approaching the seated driver. While the officer is speaking with the driver about the moving violation, the officer may see an illegal item -- drugs, a pipe, or weapons -- sitting in plain view inside of the car. Most often, drugs are seen on the center console or dashboard. Once the officer sees the contraband, in plain view, he or she may search the driver and the entire car, ultimately making an arrest based on whatever the search yields.
There are limits, however. Often the officer is observing from a viewpoint that may not be legally valid under the 4th amendment, or a violation of another 4th amendment right may be violated in order for the officer to see more clearly. For example, if in the above traffic stop example, the officer sees a baggy on the dashboard that is partially covered with a notebook; if the officer moves the notebook aside in order to view the baggy, then the act of moving the notebook could be considered a search without a warrant. Despite observing the baggy filled with marijuana after moving the notebook, the marijuana bag should be supressed at trial (not used as evidence) since the driver's 4th amendment right against unreasonable search and seizure was violated.