Knock, knock, knock.
"W-who is it?"
"Professor Quirrell, it's me, K."
"Oh, it's K. Come on in."
As soon as the muffled voice came through the door, the Defense Against the Dark Arts office door clicked open on its own. Avada walked in. "You should have given me a heads-up, Professor."
He shut the door behind him, speaking with a mixture of complaint and lingering fright. Sure enough, he felt his Occlumency come under attack once again, though the intrusion was still very slight.
After the Troll attack, all the students had returned to their respective common rooms. Once they heard the Troll had been dealt with, they relaxed and carried on with the festivities right there in the common rooms. In fact, aside from the younger students, hardly anyone had been particularly worried. As the baseline for dangerous magical creatures, Trolls were hardly impressive; any reasonably capable fourth-year or older could easily handle one. To top-tier wizards like McGonagall and Flitwick, a Troll posed even less of a threat than an owl dropping feathers into their breakfast.
So, with everyone calm and the mood quite relaxed, people soon returned to normal life. As soon as the party was over, Avada left the Hufflepuff Common Room and went to the third floor to knock on the door of the Defense Against the Dark Arts office.
"What's the problem? It was only a Troll. Even you could have handled it with ease, couldn't you?"
Quirrell—or rather, Voldemort now—said with a genial smile, secretly using Legilimency to "read" Avada's thoughts.
"But there was a first-year student there! The Troll nearly beat her to death!"
Avada looked worked up. "I can't even imagine how terrible it would be if a student were killed at school by a Troll that appeared out of nowhere!"