This has probably already been found, but the only way someone can find you is if they're chamming (or if they know the spot) You are one hundred percent hidden.
Note: This glitch works as a human or an infected!
Go to the board under the flight of stairs that you use to get to the office safe room.
Go to the board.
Sprint at the board, and you're in!
Move a little to the left to stay perfectly hidden!