NOTE: This is during Chapter 3: Enemy of the State
There is an alarm switch in the basement of the Government building. After you stealth kill all of the guards, head to the front desk (where you awkwardly stealth killed the guard sitting in the chair) and face towards the front doors.
From here, head left into the corridors and open the double doors. Head around the elevators and through the blue door. Go down the stairs and head left. The switch is in the 2nd room on the left. Head up and unlock the safe.
If you see a flashing red light above the safe, then you didn't disable the alarm correctly. The achievement will pop as soon as you jump back over the fence.
Source: Cougar24 - trueachievments.com