Su Er accepted the little task Dumbledore had assigned him. He also understood the implication: Dumbledore did not want anyone else to overhear the upcoming meeting, and wanted him to leave as well.
As he approached the door, Su Er glanced at the ear hiding in the shadows, and an idea for a prank suddenly came to him.
He looked back at the people in the room, drew his wand, and lightly tapped himself.
"Nonverbal spellcasting. Not bad." Alastor Moody was somewhat surprised by Su Er's practiced wandwork, and his lingering unease eased a little.
As for whether Su Er's use of magic here would expose him to the Ministry of Magic's monitoring, aside from the Fidelius Charm surrounding the place, the Trace had another property that practically every wizard knew about-it would be blocked when an adult wizard was present.
Dumbledore also noticed Su Er's figure gradually disappearing. The corners of his mouth curved in amusement, and he continued the meeting.
"Amelia wrote to inform me..."
Su Er was up to mischief. He wanted to scare the two rats eavesdropping upstairs, so he tiptoed over, gently opened the door a crack, and slipped out.
The stairs were wooden, so he had to be careful and move as lightly as possible. After all, "rats" had very keen hearing.
The brothers George and Fred were lying in the corridor at the top of the second-floor stairs, their bottoms sticking up and their backs to Su Er.
From their position, they could not see that the door downstairs had opened a crack. They were fidgeting anxiously with something shaped like a little horn. It seemed they were wondering whether some small problem with their "trick" was preventing them from hearing anything, but they did not dare poke their heads out to see