The following excerpt is from US v. Rousseau, 257 F.3d 925 (9th Cir. 2001):
In considering this matter we initially observe that evidence in "plain view" may be seized without a warrant, so long as the initial intrusion leading to the view is lawful and the incriminatory nature of the evidence is immediately apparent to the officers. See Horton v. California, 496 U.S. 128, 136-37, 110 S. Ct. 2301, 2308 (1990). Here, of course, the knife was in the officer's plain view from outside of the truck and its incriminating nature was immediately apparent.
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