佢將pencil芯插在門na,又把門柄而下少少,when someone go in ,pencil芯會斷,門柄亦不會保留而下少少(平常when he open door 還可拉上少少, but if someone went in , the "someone" don't know this ,and will 把門柄拉到最上 cant ),so,夜 神 月知someone went in,but the room was still the same 樣子, 所以夜 神 月 knew that it wasnt his mum or sister went in, but someone 有意監 視他的人,so he knew dat there was 竊聽器 in his room...