In the hallway where you first meet the red charging monster, you will find a shower room in the left.
At the wall corner where the soap lays down, you can go through the wall in the left side.

At the final part, where you meet a shotgun guy as you open the door, you can go through the wall of your right side.
Note that I accidentally found this secret while dealing with the enemies protecting the exit elevator.

At the final part, where you deal with 4 rocket launchers guys, there's a fake wall/TV screen in left side of the room.
Right next to that plant pot in the middle.

And the final secret it is in the room with Yung Venuz's portrait, behind the DVD trophy of portrait's right side.