The fundamental requirement is that there should be reasonable grounds for a search. The police need not apply for a search warrant, even if they have grounds to obtain one. The opinion of the Court, delivered by Alito J and joined by Roberts CJ, Scalia, Thomas, Kennedy, Breyer, Sotomayor and Kagan JJ (Ginsburg J delivered a dissenting opinion), mentioned five reasons for not requiring a warrant. They are set out at p 12 of the slip opinion. I summarise:
- The police may wish to speak to the occupier to see if it is worth getting a warrant.
- The police may want to ask the occupier for consent because that would be easier.
- They may want more evidence before submitting a marginal application for a warrant.
- They may want evidence to justify a broader warrant than they currently have grounds for.
- They may not want to disclose that they have grounds for a warrant because that might alert other suspects.
Nor is it appropriate to pretend that the police may always behave as if they were private citizens as far as door-knocking is concerned, as the majority do at p 16. Private citizens are not state agents collecting evidence to prosecute suspected offenders. It is disingenuous for the Court to hold that occupiers may "stand on their constitutional rights" and not answer the door or allow police entry, but if they "elect to attempt to destroy evidence [they] have only themselves to blame for the warrantless exigent-circumstances search that may ensue."
One would think that, on this approach, a refusal by an occupier to allow police entry would be understood by the police to be an attempt to preserve an opportunity to destroy evidence, and would therefore be justification for a warrantless entry and search.
The opportunities for abuse of powers in this context are such that, for people who regard the Fourth Amendment as a proper restraint on executive power, Ginsburg J's dissent may be considered the sole voice of reason in this case.
But the case is not about random door-knocking by the police. The police believed that the smell of cannabis was coming from a specific apartment.
The ratio of this case is that in deciding whether a warrantless search was reasonable the court will need to consider all the circumstances, and unreasonable police conduct does not give rise to reasonable grounds. This is unobjectionable as a legal rule. If it had been unreasonable for the police to knock on the door in this case, the police would not have been able to rely on subsequent inferences they drew as to risk of destruction of evidence (the exigency). There was no evidence here (pp 17-18) that the police had acted unreasonably prior to entry, so (p 19) the exigency justified the warrantless entry. This was on the Court's assumption, for the purposes of argument (p 17), that there was in fact an exigency here. This assumption was only necessary if, without the exigency, there would have been inadequate grounds for entry. So the police here had reasonable grounds for knocking on the door but, at that point, not for entry, but when (if) the exigency occurred they had (would have had) reasonable grounds for entry. The case was remanded to the Kentucky Supreme Court for further proceedings not inconsistent with the Court's opinion.
A strange thing about this case is why exigent circumstances were relevant. If the police smell cannabis ("marijuana") smoke and establish that it is coming from private property, they would have reasonable grounds to believe that an offence was being committed and so could search without warrant: for example, R v Gurnick  NZCA 19, (1999) 16 CRNZ 513 (CA). But the position is different in the US, where there is a stronger preference for grounds to be assessed by a neutral official: Johnson v United States, 333 U.S. 10 (1948). In that case the smell of opium from a hotel room, without the risk of a suspect fleeing, or of destruction of evidence, was not grounds for a warrantless search.