In Criminal Procedure, the "automobile exception" is one of the exceptions to the "requirement" that a person may not be searched, or his possessions seized, without a warrant. The history of the Fourth Amendment shows that the point was not so much to require a warrant, but to make sure there is probable cause. The warrant requirement is designed in part to require pre-enforcement judicial review. However, there are certainly situations in which obtaining pre-enforcement judicial review may unreasonably hinder law enforcement. The 20th Century has been particularly productive in terms of the development of exceptions to the so-called "warrant requirement".
The "automobile exception" to the "warrant requirement" states that when an officer stops a vehicle and arrests the driver, he may search the passenger compartment, including all containers even if closed. New York v. Belton (1981).
Now, in Arizona v. Grant (2009), the United States Supreme Court pares back the scope of permissible searches.
There's just one problem.
According to the Supreme Court, inventory searches, as administrative proceedings, do not require probable cause. So long as an inventory search is conducted in good faith and follows a standardized procedure, it is permissible, and evidence turned up during such a search is admissible against a suspect in court.
So, if police want to search your car, and they already have enough probable cause to arrest you, all they have to do is arrange for your car to be transported to an impoundment yard, where a very thorough inventory search, for the purpose of making sure that everything is accounted for when you are released and get your car back, is then to be conducted.