Okay mrdrnose, you said that the unknown achievement was bound in RUST and required the red key to obtain it, so MIX and I assumed it had to do with visiting and leaving the RUST labyrinth alive, just like the bunker visit achievement, but no, returning to the school from RUST does nothing, unless you wanted us to do something extra like use the knife or fork on the door behind the red key RUST door, but other than that, we're still lost on this achievement.