Shuichi: “The culprit used that to hit the knob of the sliding lock to lock the door.”

Maki: “So the gold leaf was on the sliding lock because..”

Shuichi: “Because when the katana hit the lock, some of the gold leaf rubbed off on it. The gold leaf on the katana did peel rather easily.”

Monokuma: “Hmmm… That evidence rings a bell. Specifically, a school bell…”

No one had no idea what Monokuma was talking about, but ignored him.