As I said below Jane Austen is not a haphazard writer. She is quite deliberate with what she tells her readers so I was quite caught by this simple sentence in Chapter 20, relaying the history of Miss Fairfax: "With regard to her not accompanying them to Ireland, her account to her aunt contained nothing but truth, though there might be some truths not told."
It is is only a sub-clause of a sentence but it seems a most important sub-clause. I don't think Jane would mention some truths not being told, unless Miss Fairfax was withholding something from her family. So what is Miss Fairfaxes' secret? Could Emma's supposition about Mr. Dixon be correct?