The touchstone of the Fourth Amendment is reasonableness. The Fourth Amendment merely proscribes those searches which are unreasonable. Courts have long approved consensual searches because it is no doubt reasonable for the police to conduct a search once they have been permitted to do so.
The standard for measuring the scope of a suspect's consent under the Fourth Amendment is that of 'objective' reasonableness -- what would the typical reasonable person have understood by the exchange between the officer and the suspect? Illinois v. Rodriguez, supra, at 183-189; Florida v. Royer, 460 U.S. 491, 501-502, 75 L. Ed. 2d 229, 103 S. Ct. 1319 (1983) (opinion of WHITE, J.); id., at 514 (BLACKMUN, J., dissenting). The scope of a search is generally defined by its expressed object. United States v. Ross, 456 U.S. 798, 72 L. Ed. 2d 572, 102 S. Ct. 2157 (1982).