The Mirror of Erised
I have looked both on Wikipedia and Hp-Lexicon, and it does not say where the Mirror of Erised is after Harry Potter and the Philosopher's Stone.
Dumbledore informs Harry that it is to be "moved to a new home" but we later find that it is used as the final protection for the Stone. It is not featured at all later in the books.
Would I have free reign to put the Mirror back at Hogwarts? Perhaps, once again, in an abandoned classroom?
I probably wouldn't put it in an abandoned classroom, where it can be found by anyone, because Dumbledore said the Mirror itself has its own dangers; that men have even gone mad sitting in front of it. So you wouldn't want it somewhere even vaguely insecure.
I'd say keeping it in the same chamber where the Stone was hidden is probably a good idea, since it's not likely to have been moved to the Chamber of Secrets.
Hmm. That's a good idea.
My story takes place during Prisoner of Azkaban, and I wanted my OC to happen upon the Mirror by accident. Hm. I might have to go a bit out-of-canon on that bit...
Well, you could have heard the student overhearing (or having passed down) about how the Trio got into that and saw *gasp* The DARK LORD(!!) in the school! And possibly the student wanders through those corridors vaguely thinking about checking it out. Because to get to where that mirror was, they'd have to go through the trap door and through several other rooms, even if the other protections had been removed.
Unless they just moved the mirror to that room in the third-floor corridor. You could do that without horribly breaking canon.
I think the third-floor room would work.
Haha, and my OC is the Ancient Runes teacher.
Hmmm. Isn't it "odd" that there was no protection from the Ancient Runes, Astronomy, or Arithmancy teachers? >_>
Is it beyond the bounds of possibility that it might be in the storage vault of the Room of Requirement? So many other things end up in there, and so few students (until OotP anyway) know about it, might Dumbledore have placed it in there? I'd have thought it was as good a place as anywhere else for it. Personally, I always imagined the Stone rooms to have temporary in nature, rather than something set in stone - no pun intended!