"In sum, a reasonable officer would not believe that a
parolee’s consent to submit to search on demand eliminates
the need to make such a demand, absent an exigency
or demonstrated futility." Green v. Butler, No. 04-2993, slip op. at 17 (7th Cir. Aug. 24, 2005). In Butler, agents went to search a parolee's residence. The parolee was renting from friends, though, and thus when the police barged into the house without knocking or announcing, the family sued. The language in the case is much broader, though, and provides not just that someone with an expectation of privacy in his home be given notice before police officers barge in, but that parolees (who have no expectation of privacy) must also be warned before the police enter.
In explaining the importance of the knock and announce rule, Judge Ripple wrote:
More importantly, the entry alleged presented significant dangers for the officers, who, in entering unannounced, exposed themselves to the risk that an occupant would mistake their entry for an invasion and reasonably would take defensive measures to protect himself from the perceived, though mistaken, threat.
In the same vein, observance of the knock and announce rule is a significant safeguard to the occupants of the home, including innocent third parties for whom the surprise of an unannounced entry by law enforcement officers might elicit panic or other forms of irrational conduct—action that easily can be misapprehended by law enforcement officers and result in deadly defensive measures on their part. Specific to the facts of this case, notice of impending entry might have given the occupants a chance to control the dog, reducing the risk to the agents of an accidental attack or of the need to “shoot” the animal. [Fortunately, their beloved family dog wasn't shot.]
That's exactly right. Imagine if the homeowner's had thought the plain clothes officers had been burglars? Someone might have been killed. The knock and announce rule should be required to keep everyone safe. That is, a search can't be reasonable absent knock and announce.
As an aside, the opinion is somewhat amusing given this introduction:
The named Illinois parole agents ... entered the residence to search Belter, prompting Mr. Green and Ms. Poulsen to file this § 1983 action for violations of their rights under the Fourth Amendment.
Section 1983 actions are apparently so common that courts needn't bother with a full citation.