Sir simon went to speak to the another Ghost,early morning.how did he know that he he was fooled/tricked again?

Sir Simon decided to make another attempt at scaring the twins after he heard them go to bed. he left his secret chamber and glided through the corridors. He got a fright as he witnessed another ghastly ghost behind a corner. As he had never seen a ghost prior to this, he hurried back to his room. After giving it a little thought, he decided to speak the ghost: two ghosts were definitely better than one! That is when he realised he had been made a fool of again: it was a trick played by the twins by hanging sheets to make a fake ghost.