HROT Demo

At the end of the game in the water area with the valves is a door that requires usage of a "silver key".

See here:

The key is located inside of the church. Backtrack to here and go up the ladder to the button that lifted the platform before. Press the button to lower the platform. This reveals a hidden crevice with the key inside.

Go back to the door at the water-valves area and unlock it with the key. A collectible is inside; get it and the achievement alongside it.

This is located at the very end of the game in the water area with all the valves. Go to the opposite end from the graffiti to this spot:

Below the water where I am facing in the picture is a vent. You can shoot this vent to enter a small area with a secret, granting you the achievement.

It doesn't look at all like the achievement icon.