After you get the blue key card from the cafeteria, and blow up the barrels in the hallway to the north, you'll notice 4 phone booths and a soda pop machine next to the phones at the end of the hall. If you look at the phones, you'll see that one of them has a light above it broken out. Use this phone and the pop machine will swing out revealing a secret area with some missiles and a gas bomb inside.
Just inside the curtain that is unlocked by the blue access card, there is a small office area over to the right. Go up to the computer and use the keyboard and the file cabinet in the corner will lower for a short time revealing a little nook with some caltrops and a portable medkit.
Just past the second secret area, there is a red van with a palm tree in the corner. There is water behind the van. If you dive down into the water, there is a secret area to the north. There is also a Baby Ripper in there so prepare for its attacks. Inside you'll find armor, Uzi clips and a flash bomb.
As you continue around through the show room, the next vehicle you come to is a red truck. At the rear of the truck, against the wall near the tree, you can see that the wall is slightly discolored next to the tree. Push on the wall and another secret area is exposed. Inside are shotgun shells, a smoke bomb and missiles.
Right after the last secret area, you'll notice a crack in the wall just before the ramp that leads to the red convertible. Blow up the crack and a nuclear missile will fly out and an opening will expose a gas bomb and a medkit.
The next secret is found in the service department behind the red access curtain. Break the glass between the waiting room and the service stalls and jump through the window and up to the car on top of the hydraulic service rack. You will see a wall switch in the corner as you walk towards the wall. Flip the switch twice until the lights go off in the service garage. Now jump back through the window into the waiting room and flip the wall switch off in here. The bookcase against the wall will lower exposing a hidden area with a Grenade Launcher, a couple Uzi clips and a flash bomb.