Amon didn't run out of the room after breaking into the house like the seniors thought. He broke the door to create the illusion he had rushed out of the room. Instead, he went and hid inside one of the closets in the room. In their haste to not lose Amon, the seniors had instead gone past him without realizing it.
Amon remained hiding until he heard the hum. It was the same hum when Amon first laid eyes on the Pledge Demon. That's how he knew his ploy to pit two opponents had worked. He came out and went to the broken door to peek.
He watched how the Pledge Demon quickly eliminated the two seniors on the ground floor. When the Pledge Demon went for the seniors on the second floor, Amon bolted. He didn't want to risk it.
The high-pitched cry of the Pledge Demon also affected Amon even when he was far away. He felt his body become numb.
'Mental attack…,' Amon thought.