Near the middle of Sector 5, you'll walk into a symmetrical, square room with an exit in front of you, and two square areas recessed in either side. The male voice that's been coming in your radio (the one advising you that the lady is a LIAR) will come on to talk to you at this time as well. The exit is clearly in front of you, but if you instead head to the left you'll find a glitchy looking square at the top of the stairs - simply walk through this into the hidden area for this achievement.