U.S. v. Conerd - The Fourth Amendment has exceptions to the warrant requirement which sometimes render a warrant unnecessary
Numerous exceptions to the warrant requirement of the Fourth Amendment allow law enforcement to avoid having to get a warrant to do something they otherwise wouldn't be allowed to do without a warrant. One of these exceptions is called the "emergency aid" exception. The easiest example of this exception is when someone is injured, they call 911, and the police respond. If they get to a house where someone needs help and no one answers the door, the police can kick in the door to render aid. If they just so happen to see drugs in plain view once they enter the house, they can seize the drugs and arrest whomever they have probable cause to believe possessed those drugs.
Here, a 911 call was placed informing law enforcement that an assault was ongoing in the basement of a home. When police arrived, they recognized the home as one owned by a felon who was known to possess weapons and to have security cameras aimed at the front door. Seeing the only light in the house that was on was in the basement, the officer went up the side of the house to view into the basement window and avoid being seen by the camera at the front door. Once he looked in he observed the defendant smoking from a drug pipe, but no assault taking place. The officer left, got a warrant to search the house based on what he'd seen, and came back and made an arrest.
The defendant argued it was an illegal entry onto what is known as the curtilage of his home. "Curtilage" is the area surrounding one's property that is close enough to a residence that a person would have a reasonable expectation of privacy in that location, like a porch. Recent caselaw has provided more protection of one's curtilage, so the argument was reasonable. However, the court ruled that due to the 911 call alleging the possible ongoing assault of an individual, the police had a reasonable belief that an emergency existed and that someone was in need of police aid. This made the entry onto the curtilage legal, and once there the additional scene observed was enough probable cause to get a warrant for further entry into the home.
While warrants are preferred, they aren't always obtained. If you suspect a Fourth Amendment violation led to your arrest, talk to a qualified defense attorney immediately.