This place is filled with parked cars which are not all police cars. There is a manhole on the southwest area that can be used to get underground.
There is one police car with a 7439 license plate that has the JMB Hp3 handgun for Claire and the Gun Stock (Matilda) for Leon inside its trunk, in order to open the trunk, the Car Key located at the outside part of the Firing Range needs to be used by inspecting the key near the car and pressing its remote control on the back.
For Claire's scenario, opening the Parking Garage exit will trigger an event where the Tyrant appears to pursue the player. In Leon's scenario, since Tyrant has already shown up when Leon got the garage key card in Jail nearby, arriving here will trigger a cutscene where Ada puts the Tyrant out of combat.