In canon, Quirrell had been a Hogwarts professor for several years before Harry enrolled, and other professors actually had noticed that he hadn't been himself ever since he came back from a trip. Specifically, he had suddenly become unusually meek and afraid of everything. They attributed it to something like post-traumatic stress syndrome; I don't remember the details, but they seemed to believe that he had encountered some kind of danger and had barely escaped with his life. (In Book 7, it's mentioned that Dumbledore had indeed been suspicious of Quirrell and had given Snape the task of watching him.)
You're right! Fixed.
[Update: and now there's a fifth discussion thread, which you should probably use in preference to this one. Later update: and a sixth -- in the discussion section, which is where these threads are living for now on. Also: tag for HP threads in the main section, and tag for HP threads in the discussion section.]
The third discussion thread is above 500 comments now, just like the others, so it's time for a new one. Predecessors: one, two, three. For anyone who's been on Mars and doesn't know what this is about: it's Eliezer's remarkable Harry Potter fanfic.
Spoiler warning and helpful suggestion (copied from those in the earlier threads):
Spoiler Warning: this thread contains unrot13'd spoilers for Harry Potter and the Methods of Rationality up to the current chapter and for the original Harry Potter series. Please continue to use rot13 for spoilers to other works of fiction, or if you have insider knowledge of future chapters of Harry Potter and the Methods of Rationality.
A suggestion: mention at the top of your comment which chapter you're commenting on, or what chapter you're up to, so that people can understand the context of your comment even after more chapters have been posted. This can also help people avoid reading spoilers for a new chapter before they realize that there is a new chapter.