First of all, see Lights Puzzle
Here is when this achievement starts; near the start of the game there is a panel in a wall, with two lamps above, and when the electricity goes off the panel falls revealing a switch (and the lamps become black). If you pull that switch, in another room nearby a ladder drops from a hatch, go up and guess what: you find a wisdom gem. It is not collectable, only decoration. A text appears: "Secret unlocked: Submachine Fan". This would be a tribute to the secret that is in the latest versions of Submachine 1.
This last one does not convince me much, since I don't know if Mateusz will approve it (though some other games like Deep Chalk have a wisdom gem), but if it's done it would be great!