Aaaand we come to the Halloween Troll surprise.
As in canon, Quirrell burst into the Great Hall during the feast gasped out that there was a Troll in the dungeons and fainted. And as in canon, Albus sent all of the students back to their common rooms.
Completely dismissing the fact that the entrances to the Slytherin and Hufflepuff common rooms were *in the dungeon*. This time he got called on it, and reversed his order. The students should stay in the Great Hall, and he and some of the staff would go and deal with the Troll.
In the confusion, Harry saw that Quirrell was no longer in the Hall. He was sure that Quirrell was after the stone, and ran off to the clubroom to check the peephole. Draco caught up to him and went with him.
There was no Muffy in the clubroom. When called she told them that Elves have their own way of keeping track of the professors. She popped away before they thought to ask her where Peofessor Quirrell actually was.
They hung about for a while checking the peephole, seeing no sign of Quirrell. Eventually they became bored and started back to the Great Hall.
And, of course, ran right into the Troll.
Meanwhile Hermione had noticed that they were gone and told Professor Burbage, who went after them.
She caught up to them not long after the Troll did and tried to fend it off. (She isn't a lot more effective at defense than they are.) Which is just about the time that Snape, who had also gone to check on the forbidden 3rd floor corridor came upon al of them, and dealt with the situation.
In 2011,
And in 2020.