Prohibited game...Have Fun

First of all, this game requires four players. Play this game around midnight period(11:30-12:00), and this game must be carried out in a rectangle-shaped, empty and locked room. Before the game starts, all lights must be turned off, and each player goes to his/her respective corner in the room. Each one must face the corner, and must never try to turn his/her head backwards during the game.

Start. The first player will walk slowly to any corner beside him(but not the corner opposite him). When he reaches the corner, he will pat his friend's shoulder. The one whose shoulder was patted will then walk to another corner, in the same direction of the previous player who patted him. The same goes on. Now think about it. The last player(the fourth) will arrive at a corner with no player. He will has to cough loudly(so that all other players can hear his sound), and then proceed to the next corner, following the same direction, until he arrives at a corner with a player.
Note that the direction can be either clockwise or anti-clockwise.

The interesting part is here. When the time is 12 sharp, an odd thing happens. The game is only normal when you could still hear your friend's coughing sound, but when 12 arrives, the room will be as silence as a graveyard. You no longer hear people coughing, but only footsteps.

How could this happen? The only explanation given by players who experienced this game before was - an unknown player sneaked in during the game.
So, who was the extra player?