Probable cause. :)

"There are times when police can perform a search without a warrant, and most searches actually do occur without warrants being issued.

If there is a reasonable expectation of privacy and there is not probable cause, a search warrant is required.

However, if probable cause does occur, such as a suspect runs away, a gunshot is heard from another room in a home, or even when an individual makes a sudden movement, a search becomes legal without a warrant."

https://www.legalzoom.com/articles/know-your-rights-can-you-...

There are exceptions to warrantless searches and entry given probable cause, reasonable suspicion, and iminent public threat. If a guy is doing 120MPH down Santa Monica Boulevard after emptying a magazine at a Girl Scout who was out of Thin Mints, that's gonna check a whole lot of "the rules in this situation are relaxed" boxes.