In many cases, to search a residence or make an arrest, the police need to have a warrant. Citizens are protected in this way by the Fourth Amendment. However, knowing that the reality of police work means that warrants cannot always be given out in time, there are exceptions to this rule. A few key ones are noted below:
1. Police can search a person who has been arrested without a warrant, as long as that arrest itself was legal. This is largely done for police protection. If they arrest a person for theft, for example, they can pat him or her down to make sure the person doesn't have a weapon.
2. When a person is arrested for allegedly committing a felony offense, the police don't need a warrant if it's done in a public place. All they need is probable cause to think the person was in fact responsible for the crime. For example, if police get a call about a shooting and they then see a person running down the street who matches the description of the shooter, they can make an arrest without a warrant.
3. If a suspect is fleeing, the police can sometimes make an arrest even in a non-public area. Typically, to go into a home or apartment, the authorities have to have a warrant, but this is a key exception. It is in place to keep someone from running from the police, running into the house, and then being safe from arrest merely because they're on the other side of the door, even though the police still have probable cause for the arrest.
While these are just a few examples, they show how warrants certainly aren't needed in all situations. It's crucial to know when they are and when they aren't to determine if your rights have been violated during a search or arrest in California.
Source: FIndLaw, "The Fourth Amendment "Reasonableness" Requirement," accessed June 24, 2016