How Did She Know?

I believe I’ve found what has to be the most completely insignificant plot hole in the Potter series and--for this--I feel I should be commended.

Half asleep, I connected the problem while watching the Sorcerer’s Stone DVD and, sure it was a screenplay oversight, consulted the book straightly way. But, alas! It is exactly the same in the book. Let’s see what you make of it:

In Chapter Ten, “Halloween,” of Philosophers Stone, after Hermoine reproves Ron for his failed swish-and-flicking, she overhears Ron insulting her in the crowded corridor. The book says Hermoine didn’t attend their subsequent class and wasn’t seen all afternoon (is this the only account of Hermoine skiving off class of her own accord?). It goes on to say, “Harry and Ron overheard Parvati Patil telling her friend Lavender that Hermoine was crying in the girls’ bathroom and wanted to be left alone.” Harry and Ron go to the Halloween feast where they seem to be enjoying themselves until Quirrell comes in announcing the presence of a troll in the dungeon. While the different houses are being led back to the common room by the prefects, Harry suddenly remembers:
“I’ve just thought -- Hermoine.”
“What about her?”
“She doesn’t know about the troll.”

Okay, so Hermoine doesn’t know about the troll, right? The troll wanders around for a while and so happens to find Hermoine where, evidently, she has been all day-- in the girl’s bathroom. So Harry and Ron save Hermoine via the old “I’ll stick my wand up his nose and you bust a ‘Wingardium Leviosa’ on his ass” trick. It works and Hermoine is rescued. When McGonagall, Snape, and Quirrell arrive, the trio is standing around an unconscious troll. When asked what they are doing there, Hermoine pipes up and offers the excuse that she went looking for the troll thinking she could handle it because she’s read all about them.

Here’s the inconsistency with this. If Hermoine has been in the girl’s lavatory all day, how did she know that the announcement of the troll’s trip inside the castle had been made? Eh? EH?

Well, there you have it. Pretty insignificant, huh? If you ask me, it is not a hole. I think it is foreshadowing Hermoine’s natural propensity for Legilimency.

Thank you!