It's been long patched. But basically, you had to go to a door that was about to open, and wait until you are about to hear the alarm ring for the fifth time, then you toss your grenade at the tip of the upper right corner of the rectangle box in the middle of the door, and the grenade would get stuck inside and glitch the door. You can still do it on one of the broken doors on the bottom floor, even though it doesn't do anything to it.