To constitute a valid consent to search, the consent must be given “freely and voluntarily.” If a police officer wrangles consent through trickery or coercion, the consent does not validate the search. (But see Is police deception always legal?) Often, a defendant challenges a search on the grounds that consent was not voluntary, only to have a police officer testify to a conflicting version of events that establishes valid consent. In these conflict situations, judges tend to believe police officers unless defendants can support their claims through the testimony of other witnesses.
Example: Assume that Officer Mayer knocks on Sue's door and asks for permission to search her house. Before Sue consents to Officer Mayer’s entry into her home, Officer Mayer falsely tells her, “It will do you no good to refuse me entry. I’ve got a warrant, so I’m prepared to come in whether or not you consent.” Sue replies, “If you’ve got a warrant, I might as well let you in. Look around all you want.” In these circumstances, because of the officer’s false claim of having a warrant, Sue's consent to the search wasn’t voluntary. However, it may be Sue’s word against the officer’s as to whether the officer tricked her into consenting.
Example: Undercover cop Jones, posing as an employee of the gas company, asks Casey to allow him into Casey’s home to check for an alleged gas leak. Casey agrees. Jones enters and sees drugs and drug paraphernalia in the kitchen. Jones’s entry into and search of Casey’s home is invalid. Consent that is obtained by fraud is not considered voluntary, and Jones’s story that he was a gas company employee is fraud. (But, again, see Is police deception always legal?.)
Example: Same case, but this time Jones has been posing as a parent in Casey’s son’s school and has made friends with Casey independent of his undercover mission. Casey invites his “friend” Jones in to play cards. Once inside the home, undercover agent Jones unexpectedly sees illegal drugs. He seizes the drugs and arrests Casey. In this situation, under the law, Casey was not tricked or coerced to let in Jones. He just didn’t know who his friend really was. The Constitution does not protect people from the consequences of having what courts call a “false friend.”