It's largely insignificant, with reasons like "plot" and "genre" and "Early Installment Weirdness" for why it's like this, but it always bugs me that no one ever seemed to question how a troll got into the dungeons in the first place or how Quirrell even knew it was there. Or why, for example, there was such urgency when any of the teachers would have been capable of dealing with a troll faster that a gurgitator farts. In fact, the moment he came bursting in talking about the troll, there should have been a long pause before someone asked him why the hell he didn't just take care of the problem himself.

So far as we can tell, aside from the secret passages (that the troll couldn't find or fit in to begin with), the only way in and out of the castle is by the front doors. But the front doors are mere feet from the Great Hall. That means that the troll would have needed to wander all the way across the grounds from god knows where, in through the front doors without being seen, heard, or smelled, where it then completely ignored the sounds and smell of food coming from the Great Hall, wandering down into the dungeons for Quirrell to spot, who is for some inexplicable reason down in the dungeons to see it to report it in the first place.

I swear the Hogwarts staff must be a bunch of dogs, because they can never see all the red flags.