This time around I payed special attention to the hidden clues about Frank/Jane relationship and wondering how much JA wished to reveal and conceal.

The first such incidence is when Emma questions Jane Fairfax about Frank Churchill. Is Jane's reserve here unnatural considering how everybody is curious about Mr. Weston's son?
The like reserve prevailed on other topics. She and Mr. Frank Churchill had been at Weymouth at the same time. It was known that they were a little acquainted; but not a syllable of real information could Emma procure as to what he truly was. "Was he handsome?" -- "She believed he was reckoned a very fine young man." "Was he agreeable?" -- "He was generally thought so." "Did he appear a sensible young man; a young man of information?" -- "At a watering-place, or in a common London acquaintance, it was difficult to decide on such points. Manners were all that could be safely judged of, under a much longer knowledge than they had yet had of Mr. Churchill. She believed every body found his manners pleasing."

The second incidence is when Frank makes long explanation about paying visit to Jane Fairfax the first time in Highbury, when nothing should be more natural. LOL about Emma's observation at the end!
"As you are going farther on business, sir, I will take the opportunity of paying a visit, which must be paid some day or other, and therefore may as well be paid now. I have the honour of being acquainted with a neighbour of yours, (turning to Emma,) a lady residing in or near Highbury; a family of the name of Fairfax. I shall have no difficulty, I suppose, in finding the house; though Fairfax, I believe, is not the proper name -- I should rather say Barnes, or Bates. Do you know any family of that name?"......... "I have heard her speak of the acquaintance," said Emma, "she is a very elegant young woman." He agreed to it, but with so quiet a "Yes," as inclined her almost to doubt his real concurrence; and yet there must be a very distinct sort of elegance for the fashionable world, if Jane Fairfax could be thought only ordinarily gifted with it.

The third incidence is Frank's rather rude disparaging remark about Jane Fairfax.
"Perhaps it is as well," said Frank Churchill, as he attended Emma to her carriage. "I must have asked Miss Fairfax, and her languid dancing would not have agreed with me, after yours."

The fourth incidence is in connection with the fact that only two dances were danced at Coles' when Frank the next talks about giving everything for more! And nobody thinks it strange?
"If you are very kind," said he, "it will be one of the waltzes we danced last night; -- let me live them over again. You did not enjoy them as I did; you appeared tired the whole time. I believe you were glad we danced no longer; but I would have given worlds -- all the worlds one ever has to give -- for another half hour."

The fifth weird incidence is exchange between Emma and Frank at the ball when I can not understand why Emma thinks Frank is ungrateful, but it certainly sounds like the two of them were referring to different things.
"Jane!" repeated Frank Churchill, with a look of surprise and displeasure. "That is easy; but Miss Fairfax does not disapprove it, I suppose." "How do you like Mrs. Elton?" said Emma in a whisper. "Not at all." "You are ungrateful." "Ungrateful! What do you mean?" Then changing from a frown to a smile -- "No, do not tell me -- I do not want to know what you mean. Where is my father? When are we to begin dancing?" Emma could hardly understand him; he seemed in an odd humour.

The sixth incidence is when Frank declares at Donwell Abbey "such a point had been made of my coming! " and one has to wonder by whom. I guess here we might just see his behavior as that of a spoilt brat.

So at which point can we reasonably suspect there is something between Frank and Jane?