An officer does not need a search warrant if an individual gives him consent to search. However, the consent must be voluntary and not coerced.
“Conversely, if under all the circumstances it has appeared that the consent was not given voluntarily—that it was coerced by threats or force, or granted only in submission to a claim of lawful authority—then we have found the consent invalid and the search unreasonable.”
Schneckloth v. Bustamonte, 412 U.S. 218, 233 (1973).
“Implicit in all of these cases is the recognition that knowledge of a right to refuse is not a prerequisite of a voluntary consent. If the prosecution were required to demonstrate such knowledge, Davis and Zap could not have found consent without evidence of that knowledge. And similarly if the failure to prove such knowledge were sufficient to show an ineffective consent, the Amos, Johnson, and Bumper opinions would surely have focused upon the subjective mental state of the person who consented. Yet they did not.
In short, neither this Court's prior cases, nor the traditional definition of ‘voluntariness' requires proof of knowledge of a right to refuse as the sine qua non of an effective consent to a search.”
Schneckloth v. Bustamonte, 412 U.S. 218, 234 (1973).