This Secret Achievement is Called 'Ghost' - 15G
You need to make it through an entire hostile area without so much as a squeak.
so during the first combat area after you climb down the ladder and are prompted with a combat tutorial you will need to wait for the guard to finish on the phone then crouch and walk behind him and press B (you can move his body if you like just to be safe) then just along from the guard is a Crate on a forklift you need to pick that up and place it next to the containers so you can jump on top of them. timing is key so you don't get detected, when you get to the top you should see a ladder in which you need to climb, once at the top you need to move the Crate from in the corner so you can pass behind the electric generator, finally switch off the power and go round the corner through the vent and if you have done this correctly you should have the Achievement :)
Best of Luck