Most Americans understand that police officers sometimes need a warrant to search for information believed to be related to criminal activity. They may also be aware that, according to the exclusionary rule, evidence gained in an illegal search cannot be used to convict someone of a crime. However, in many circumstances the police do not need a warrant for a search, or for the evidence gained from a search, to be legal and used in court.
In each of the situations below, a police officer does not need a search warrant to conduct a search.
■If an individual voluntarily consents (agrees to) a search, no warrant is needed. The key question in this kind of search is what counts as a voluntary agreement? In order for a consent search to be legal, the individual must be in control of the area to be searched and cannot have been pressured or tricked into agreeing to the search.
■A police officer that spots something in plain view does not need a search warrant to seize the object. In order for a plain view search to be legal, the officer must be in a place he has the right to be in and the object he seizes must be plainly visible in this location.
http://www.streetlaw.org/en/Page/342/When_is_a_Search_Warrant_Not_Necessary