Hidden Achievement?

[]

Before the achievement was hidden it mentioned a secret on the 3 Letter Triad, has anyone found out what it is/gotten the achievement?

[Charles]

Click on the achievement panel in Steam, then go to Global, you can see all hidden achievements there.

[Maldo Straken [FR]]

Text of the achievements : " Justice Uncovers Depths Ghosts in Elysium"
Discover the location of the secret MLO meeting room