When fighting a guard and near the Assassin's Bureau the message "The Assassin's Bureau is close" will appear while you are fighting. Make the guard follow until both of you are standing on the bureau's closed entrance. Kill him and make sure he is standing on it. The guard will fall down inside the bureau and will be visible. However, he will be dead. If you are lucky when killing the guard, he will fall on the fountain and will be hanging in a random strange position while water is pouring on him.