The room itself (shown here in red) is actually just a small chamber which rests inside of a much larger one. It is mounted to the walls, floor, and ceiling of the larger chamber by a set of expertly crafted rails, which have been treated with a magical oil which does not degrade over time. So perfect are the mechanisms which move this room, that it does not produce the normal rumbling vibrations which would normally be a dead giveaway that something was amiss. Those who are particularly in tune with the ground (such as dwarfs) should be given a chance to notice the movement, but only when they are inside of the moving room itself. And even then, their ability to detect the movement is not certain.
Every corridor which can be used to approach this room is strewn with pressure plates. When activated, these plates will cause the room to move into position to receive the approaching characters. The pressure plates are specifically placed far enough away from the sliding room that it will have ample time to be in position before anyone is within eyesight of its doors. If these pressure plates are detected and avoided, there is a 4-in-6 chance that the room will not be in position when the players arrive, and they will instead find themselves looking into a long featureless room. If the players choose to enter this room, roll a check every few turn to determine if the sliding room moves to crush them.
The inside of the sliding room itself is almost entirely featureless and boring. It must be, or else the extremely sensitive weight detection mechanism would be too difficult to calibrate! So long as any character is inside of it, the room will move to a new position any time one of its doors is closed. If it is at the south-most position, it will move north, if it is in the north-most position, it will move south. If it is in one of the two central positions, randomly determine whether it will move north or south.
So, for example, if the red room is in the position shown above, and the players approached it from the curving corridor on the southern side of the map, then it would glide to the south-most part of the larger chamber before they arrive. If they see the room, and all enter it, then close the door behind them, the room will glide back to the position shown on the map. If they then exit it, and close the door behind them, it will not move, because there is no weight inside of it (unless they left equipment weighing at least 30lb behind). If the party’s halfling then enters the room, and closes the door leaving the others outside, they will see the room slide away with their halfling inside of it.