Emma refused to accept evidence she was Mr. Elton’s object because the idea he would presume to “raise his eyes to her” was an impossibility due to the inequitably of rank and fortune as well as his inferiorities “in talent, and all the elegancies of mind” (16). Emma feels Mr. Elton is her social inferior but she also realizes they are not compatible. I feel Mr. Knightley does not see Harriet as a potential choice for the same reasons—inequitably of rank, fortune, talent and understanding. I imagine Mr. Knightley feels he and Harriet are as incompatible as Emma believed of her and Mr. Elton so in that sense he is as immune to the idea Harriet may interpret his attentions to be that of a lover just as Emma was to Mr. Elton.

Emma was unguarded in her manner towards Mr. Elton because she was attempting to encourage him by proxy for Harriet. I am not convinced Mr. Knightley has been unguarded in his manners towards Miss Fairfax and Miss Smith. I don't think Jane has ever considered Mr. Knightley to be interested in her and Mrs. Weston’s felicity for connecting the matchmaking dots is as bad as Emma’s—neither her Emma & Frank or Jane & Knightley matches were stout. If the ladies ages are discounted it seems Mr. Knightley’s supposed attentions to Jane could just as well suggest he is interested in dear Miss Bates instead:

Escorting Miss Bates & Miss Fairfax, to whom he may have offered his arm at her aunt’s request, to Highbury (21).
He sent his carriage to carry Miss Bates and Miss Fairfax to the Coles party (26).
He asked Miss Bates to stop Jane from singing when her voice is obviously strained at the Cole’s party (26).
He gave additional apples, which Jane likes, to the Bates who has a third mouth to feed (27).

(Please add any attentions I have forgot!)

Although Harriet rejects it I think Mr. Knightley has sought a better acquaintance with Harriet on Robert Martin’s account. She reports interesting questions that “seemed to be almost asking her, whether her affections were engaged” (47) and I think such could be considered particular and unguarded but I don’t think his questions or behavior was actually particular enough to give Harriet hopes:

Harriet had been conscious of a difference in his behaviour ever since those two decisive dances. Emma knew that he had, on that occasion, found her much superior to his expectation. From that evening, or at least from the time of Miss Woodhouse's encouraging her to think of him, Harriet had begun to be sensible of his talking to her much more than he had been used to do, and of his having indeed quite a different manner towards her; a manner of kindness and sweetness! Latterly she had been more and more aware of it. (47)

It seems although Harriet noticed a change in Mr. Knightley’s behavior after they danced she did not begin to see any signs of particularity until after she had Emma’s blessing:

I am sure, but for believing that you entirely approved and meant to encourage me in my attachment, I should have considered it at first too great a presumption almost, to dare to think of him. At first, if you had not told me that more wonderful things had happened; that there had been matches of greater disparity (those were your very words); -- I should not have dared to give way to -- I should not have thought it possible -- " (47)