This achievement is gained while going down an elevator shortly after the "Garage Days" opportunity. You can get off this elevator after going down 1 floor, but instead of getting off, ride it all the way to the bottom. Once on the bottom floor, get off walk forward into the large pile of trash (seen in the picture) and open the secret door under your feet to find the secret room.
It can also be seen in the 100% full game walkthrough at 21:33: