Even though DUI stands for driving under the influence, a person does not have to be caught with the car in motion or even with the engine turned on to be considered driving for the purposes of a DUI. A person can be arrested and convicted for driving under the influence even when they are simply seated in the front seat of their car, or asleep in the front seat of their car.
In these situations, they are considered to be in operational control of their vehicle and are in the same position as a person seated in the front seat, with the ignition on, and the car in motion.
Typically, people are not arrested for DUI based on tips from civilians or non-police officers, unless there is an accident. When there is an accident, the police arrive on the scene and will perform a DUI investigation. If there is no accident and someone calls police to report someone driving under the influence that may not usually result in a DUI investigation.