Keep exploring the Parking Garage inside the Centennial building. There is a freight elevator, but it is blocked with a locked gate that has a sign "Ask Security Guard to open".  When you try to open that gate, you get a new objective: "Enter the building via the service elevator".

There is a blue van parked near the wall. It says "SxS - Security X Systems". Try to open the driver's side door but it won't budge. Open the back door. A cutscene will trigger. Be ready, as a Screamer jumps you from behind. You need to shake it off immediately, or you die and game will reload. After you shake the Screamer off, kill it. Other than that, there is not much in the van except some blood.

Near a beam, there is a toolbox with a crowbar in it, in case you need a new weapon.

There is a cop car parked a few feet away. When you try and open one of the doors you see that there is a police dog inside. There is no way, unfortunately, to let the dog out.  Open the trunk and find a Forensic Flashlight. It is better than your regular flashlight. You can toggle between Normal and UV flashlight modes. The UV mode helps uncover secret clues.

Silent Hill Downpour Centennial Building Garage Police Dog

In the passenger side seat of the patrol car, you find a mystery item:

Internal Memorandum
Ryall State Prison
From: Captain Bryan Handlay
To: Warden Glen Milton
Subject: Status Update - Internal Investigation
Per our recent conversation, my department has initiated an aggressive internal investigation into the prison guard staff. In order to keep you informed of significant developments, please note that we are paying particular attention to the activities, past and present, of Corrections Officer George Sewell. We have received an eyewitness testimony from Corrections Officer Frank Coleridge that suggests C.O. Sewell has been engaged in a number of illegal activities in the course of his duties.
Other then C.O. Coleridge's testimony, however, our evidence regarding Sewell's alleged violations remains circumstantial at this point and our investigation continues. I will keep you promptly apprised of any new findings.
Respectfully,
Captain Bryan Handlay
Chief of Guards

Go to the red fire truck. Examine the yellow hose on the ground near it.  There is a maintenance access gate nearby. Break the lock with your weapon. Go down the stairs, but the water is too deep. Next to it, there is some entry blocked with red bricks. Look through the hole and trigger a short cutscene.

Go back to the firetruck after opening the maintenance access gate, and use the hose. Murphy will take the hose and place it in the water in the maintenance access stairs, so that you can suck the water out.  Use the controls on the firetruck to start the water suction. After the water is sucked out, go down the previously flooded stairs. When you reach the end of the corridor, you find a wheelchair. On the wheelchair, you find:

Security Card
The magnetic strip looks worn out

At this point, you have to hurry out of there. The water will start rising quickly, flooding everywhere. The parking garage is also now filled with Weeping Bats. Run to the security office and use the Security Card on it, enter and close the door behind you quickly. On the desk inside this room there are some controls that will open the gate that was blocking your path to the service elevator. There is also a radio there. If you turn it on, you hear DJ Ricks saying he knows a way out of town. The song that then plays from the radio is "Silver Stallion" by The Highwaymen. There is also a cabinet in the adjacent room with a first aid kit inside, so be sure to grab it before leaving the security office and making a dash for the freight elevator.