This is a secret level accessible from Hotel Hell.
Secret Places (5)
As soon as you get out of the water and onto the underpass, turn left. Follow the road until it stops. Examine the ledge on the left for a crack. Use an explosive to blow up the crack, then jump up to the ledge and walk to the furthest window (far left in the image). Open this window to find the first secret.
There are two secrets in this snapshot. The first is a hidden door in the bookshelf (left side of image). The second is behind a crack in the wall next to the blue access card (right side of image).
This secret is in the room with the Terminator reference. Jump into either of the yellow/black striped walls to find the secret place.
This secret is hidden behind another operable window. You can see the secret place on the right side of the image. I included the pilons and sewer in the shot as a frame of reference. Once you get inside the secret place, you'll find a door with a crack. Blow up the crack and you'll discover that the last secret area connects back to the first!