I agree with you, Staphanie, that Emma's behaviour was odd if she really thought that Harriet was in love with Frank Churchill. It's really insensitive and hurtful.

I think Emma has two excuses: she was really cross and exhausted by the weather and company of the Eltons. It's mentioned that it was very hot and some people get exhausted easily by that weather, which was pointed out for Frank when he went to Donwell, and Emma in her thoughts admitted she is of similar disposition. She desperately tried to find at least some amusement, and Frank Churchill and Mr Weston were only two who obliged.

Did she - Emma - really consider it flirting though? I have looked at the part in the novel once again, and Emma admits that to others it must look like excessive flirtation, though at this point she knows he means nothing.

Emma was at her worst when they were on that trip, and she later truly regretted her behaviour. However, she focused mostly on her insult of Miss Bates, which was what Mr Knightley had reproached her for. Nobody was worried for Harriet's sake because this was frankly solely Emma's idea - that there might be a possibility of a relationship between Frank and Harrier. Therefore nobody saw anything too bad or too improper in her "flirtation", which I think was a part of Frank's performance to punish Jane Fairfax. What a scoundrel. Poor Miss Fairfax.