If a police officer has probable cause to believe the car has illegal substances in it, he may indeed search the car, including the trunk and any containers in the trunk.  Because a car is movable, searches without a warrant are often allowed.  Additionally, if you consent to a search (tell the officer they can search your car), the officer doesn't even need probable cause.  If the car is impounded, then an inventory search can also be allowed.  Later, additional searches might well require a warrant.