Lewis v. US, 385 US 206 (1966)-The government use of an undercover officer who is invited into the defendant's home to conduct an illegal drug trade without a warrant was not in violation of the Fourth Amendment. The court further added:
Graves v. Beto, 424 F.2d 524 (5th Cir. 1970)(no link)-An officer deceives a rape suspect into giving a blood sample. The officer told him that the sample was to be used to determine drunkenness. The sample, however, was used for comparison with blood found at the rape scene. The court determined that the consent was not voluntary.
Maryland v. Macon, 472 U.S. 463 (1985)-An undercover officer entered a business open to the public. He purchased two magazines that were later found to be obscene. The defendant moved to suppress the evidence claiming that the officer's entry was an unreasonable search in violation of the Fourth Amendment. The court ruled that the examination of any item displayed for public viewing in a private business does not constitute a "search".
United States v. Wagner, 884 F.2d 1090(8th Cir.)(1989)(no link)-A UPS delivery man and a police officer posing as a UPS employee were invited into the defendant's home to collect on a C.O.D. delivery. The officer smelled an odor he associated with the manufacture of methamphetamine. He left and got a search warrant. During the subsequent prosecution the defendant sought to have the warrant invalidated. He claimed that the officer's deception to gain entry to his home was an unreasonable search. The court ruled that "...one who consents to an undercover agent's entry into his house 'has no legally enforceable expectation that [the agent] is not an undercover officer.'"