What is the case law on the right of police officers to enter a person's backyard?

I do not question the right of the police to respond when they witness the commission of a crime in someone's backyard. But, this was not a case where the police observed a burglary or some other crime in progress. If they had, this certainly would have justified their entering the yard. (See People v. Duncan (1986) 42 Cal.3d 91, 98, 227 Cal.Rptr. 654, 720 P.2d 2.) It is not even a case where the officers had a reasonable suspicion the man in the yard was committing or about to commit a burglary or other crime on appellant's property. Indeed no evidence was introduced at the suppression hearing suggesting this is a case where the officers possessed a reasonable suspicion the man was committing or about to commit any crime. On the contrary, the arresting officer testified the unidentified man was detained solely because the officers did not believe he lived there and "didn't know what he was in that rear yard for...."