Not a secret: In the first room to the left at the start, there is some stairs to a green armour and two columns. And some guys that don't think you should be there. After taking the armour, check the columns. One of them carries a switch - it will briefly open a door to the yard with the blue armour. Out there, something else opens.

Don't do this if you are low on ammo.

After crossing the swamp, a bit of the wall to the right is discoloured. Walk up to it and use your "Use" key. It opens, and leads to goodies - and to the yard with the blue armour.

Continue into the room where you can see the 'exit' sign. When you cross the line to the flickering patch of light, you trigger a secret door. Go back to the swamp. A column at the back wall has dropped. Inside is a room with goodies.

After #3, cross back over the swamp. When you cross the line between shade and light in the floor at the entry to the swamp, a lift lowers, for a few seconds, inside the secret room. Run across the swamp, ride the lift and find goodies galore!