Updated May 9, 2016
Police can’t search places where you have a reasonable expectation of privacy unless they have a warrant or the search falls into one of the exceptions to the warrant requirement. Some of the well known exceptions to the warrant requirement apply when:
There are a number of lesser known exceptions to the warrant requirement. One of these, the "community caretaking" exception, applies when police find evidence of a crime while acting to protect the public from a potentially dangerous situation. Even if officers have obtained evidence through a warrantless search, it can be admissible in court under the community caretaking exception—that’s because the police were acting as community caretakers (not law enforcement) when they found the evidence.
For example, take a situation involving a car idling on the side of the highway with an unconscious person behind the wheel. If police go into the car to see if the driver is okay, they are acting in a community caretaking capacity. If they find stolen property on the passenger seat while checking on the driver, a court could admit this evidence in a subsequent prosecution of the driver: Even though the officers found the property without a warrant, they were acting as community caretakers, not investigators.
The community caretaking exception comes from two United States Supreme Court cases, both of which dealt with searches of impounded cars: Cady v. Dombrowski, 413 U.S. 433 (1973) and South Dakota v. Opperman, 428 U.S. 364 (1976). In these decisions, the Supreme Court upheld warrantless car searches where officers were acting as community caretakers: Cady involved a gun found in the trunk of a car after an accident; Opperman involved drugs found in an abandoned car about to be impounded. Among the reasons for the decisions was the lesser degree of privacy people have in their cars than they do in their homes.
(Keep in mind that the U.S. Supreme Court isn't necessarily the final authority on all privacy-related issues—states have their own laws, too.)
Since the Supreme Court decisions, state courts have reached different conclusions as to whether the caretaking exception justifies the warrantless search of homes in addition to cars.
In a Wisconsin case, a driver crashed into a pole, then left the scene of the accident. Police tracked the car back to his house. When they knocked on the door, they indicated they wanted to see if he was hurt. The driver’s brother let them in the house and led them into the driver’s bedroom, where they found the driver visibly intoxicated. Prosecutors charged the driver with operating a motor vehicle while intoxicated; the court allowed presentation of evidence about his intoxication under the community caretaker exception.
On appeal, the driver argued that the community caretaker exception didn’t apply because the police might have been acting as caretakers but were also seeking evidence of a crime. The Wisconsin Supreme Court disagreed, finding that as long as the police had an objectively reasonable basis to believe the driver needed assistance, the community caretaker exception justified their warrantless search. (State v. Gracia, 345 Wis.2d 488 (2013).)
The New Jersey Supreme Court reached an arguably incompatible conclusion in State v. Vargas, 213 N.J. 301 (2013). In that case, a landlord called 911 because his tenant had been missing for several weeks. The police entered the rental home without a warrant to check on the renter’s welfare. While they didn’t find the tenant, they found marijuana, evidence of money laundering, and illegal firearms. The trial court admitted the evidence in the renter’s prosecution.
When the defendant challenged the search on appeal, the state supreme court held that the community caretaker exception didn’t justify the warrantless search. It found that the police didn’t have an objectively reasonable basis to believe that there was a life-threatening emergency that justified entering the apartment without a warrant.
State courts aren't the only tribunals that have split on whether the caretaking exception applies to home searches—federal courts have, too. Some say that it applies only to auto searches; others have held that the exception can justify warrantless home searches.