Ah-ha! Yes... that was really well hidden. I think because that last pulley jump can be really tricky and became my main focus I did not even consider back tracking.
The 4 keys were well hidden, and that last room with the drain switch, accessed below the map, was about the last place I looked for another room.
Very satisfying to finally finish... Still missed 1 health upgrade, but that is fine.
Great stuff :)