Shortly after Mission 7: A Crack In The Slab begins, you will receive the "Timepiece" item that allows you to time travel. Immediately after getting the Timepiece, go to the far end of the corridor. Switch between past and present time to get through it undetected. You can also use it to see where enemies are in the other timeline. Stilton's Master Key is in the room at the end of the corridor. Switch to present time when you enter the room, crawl under the table, switch to past time, and collect the key to get the "Under The Table" achievement.