I bet on option 1. I think Quirrell has been expecting Dumbledore to be hiding in the mirror. He said:
[...]I saw the Headmaster missing... but for all my magic can tell me... he could be in another... realm of existence..
Quirrell could be lying, but Dumbledore being hidden near the stone seems reasonable too. Also Quirrell said (in parseltongue):
I have plan to sstop even sschoolmasster, if he appearss before uss.
Appears before us. To me the verb appear evokes images, like in a mirror, more than running into people.
This is a new thread to discuss Eliezer Yudkowsky’s Harry Potter and the Methods of Rationality and anything related to it. This thread is intended for discussing chapter 109.
There is a site dedicated to the story at hpmor.com, which is now the place to go to find the authors notes and all sorts of other goodies. AdeleneDawner has kept an archive of Author’s Notes. (This goes up to the notes for chapter 76, and is now not updating. The authors notes from chapter 77 onwards are on hpmor.com.)
Spoiler Warning: this thread is full of spoilers. With few exceptions, spoilers for MOR and canon are fair game to post, without warning or rot13. More specifically: