The police cannot just enter your house and look around. They need a good reason. Once they're in the house, they can perform a protective search of the premises to ensure the place is safe for them to be there. In this case, the Second Circuit upholds the preclusion of evidence (an illegal gun) because the search violated the Fourth Amendment.
The case is United States v. Hassock, decided on January 28. The police may perform a "protective sweep of a house where entry was made by police pursuant to lawful process." While the Fourth Amendment "is sufficiently flexible to allow officers who have an objectively credible fear of danger to take precautions to protect themselves," the police must "supply specific and articulable facts warranting a reasonably prudent officer to believe that an individual posing a danger is lurking in an area to be swept, we have found lacking an essential element necessary to justify a search under the protective sweep doctrine."
In this case, the police were looking for Hassock, who had a felony record and was therefore not allowed to have a gun. They went to his neighborhood one morning for a "knock and talk," asking neighbors if they knew where he was. The woman who was living in the back bedroom of Hassock's apartment with her boyfriend answered the door and said the police could come in and look around. The police found an illegal gun in the apartment and charged Hassock.
For those of you who do not practice federal criminal law but know that the courts have taken pro-prosecution views on matters like this, the assumption may be that the police can search the apartment in these circumstances. They cannot, at least not in the Second Circuit. As this scholarly decision makes clear, courts around the country have adopted a hodge-podge of legal doctrines governing protective sweeps, and someday the Supreme Court is going to have to fashion a consistent rule. For the time being, know this: the search of Hassock's apartment was illegal because, "although [the police] went to the Hassock apartment with a legitimate purpose -- the questioning and possible arrest of Hassock -- when Hassock did not answer the door, that purpose could not be pursued until Hassock was found. Under these circumstances, the sweep could not be viewed as a reasonable security measure incident to Hassock's interrogation or arrest. Instead, the 'sweep' itself became the purpose for the agents' continued presence on the premises insofar as they thereby searched the location for Hassock."
What about the woman's consent to search the apartment? That does not fly, either. The Second Circuit (Miner, Parker and Raggi) says that "it was not apparent to the [officers] that the young woman who admitted to the apartment was clothed with any authority that would allow her to consent to a search of the premises. Her discussion with the agents was very brief." As there was no authority for the agents to enter Hassock's bedroom (where they found the gun), "the original purpose of the 'knock and talk' thereupon became an illegitimate search for Hassock incident to no other lawful police conduct, which cannot be characterized as a protective sweep."