Because cars are mobile, police are given wider latitude for searching a car based upon probable cause. If they have lawfully stopped the car and they see or smell things that they believe indicate the presence of drugs, then they can search. They are not required to have a warrant, and they’ll often use a police dog to confirm their suspicions before searching. However, the US Supreme Court has ruled that the police can’t make a person wait for a dog to arrive for longer than it would take them to write a ticket unless they have enough suspicion to do so.
A home has special considerations because there is a higher level of an expectation of privacy. A search warrant is required for home searches except when there are exigent circumstances. Exigent circumstances mean that there is some sort of emergency going on inside a home. For example, someone could be getting hurt or destroying evidence. If there are exigent circumstances, then the police may be allowed to enter. However, there is a difference between entering a home and actually searching a home. If the police enter a home based on one reason and then they find a different reason to search further, they would need to obtain a warrant. They would not be allowed to go beyond the scope of the initial entry into the home or what’s in plain view. This is in accordance with the Fourth Amendment protections, which say that no warrant should be issued without probable cause, and that all warrants must describe the place to be searched and the person or things to be seized.