This occurs after level 3.5 in the door left of the check point passage.
If you have played their previous game, Pneuma: Breath of Life, you would be familiar with the point of view puzzles.
There is a camera on the left side of the room that activates only once you are not in direct line of sight. You see this as you walk in, the door on the other side of the room closes once the camera notices you.
All you need to do is use the 3 given power cubes and stack them up on top of each in front of the camera eye so that you can't be seen when entering the door on the other side.
Enter this door and click
on the computer. The achievement then unlocks.