Inside the red doors, you will find the blue key at the edge of a toxic waste pit. Grab the key, jump in the pit, and run to the right to get a protection suit. Carry on to the hole in the wall for goodies.

From #1, proceed to the only exit, before the suit run low. When you reach the exit, you will be credited with another secret. It makes some sense, as you can reach this one without finding #1.

Not a secret: Inside the maze beyond the blue door, nearly as far away as you can get from the door, is this 'secret' door: misaligned texture, diverging color, and a arrow made out of helmets pointing to it...
Secret? No way!

Next to the yellow key, there is a secret door. tap the wall, goodies inside. Technically, you can reach this one from outside the blue door, but as you have to wade through toxic waste without protection, it is not adviceable.

After you have found the previous room, slip into something more comfortable... ehhh, the hazmat suit! Wade through the slime, cross the gangway and continue down the waste channel on the other side. Goodies, and access to a otherwise unaccessible yard.