If Mr Knightley 'knows' something about Mr Elton's proposal to Emma, that may, at least in part, be due to conversation with his brother John, who noticed that Mr Elton's attentions to Emma were becoming rather particular. This would be consistent with what Mr Knightley already knows, that Mr Elton hopes to marry a woman of fortune.

As to Miss Bates' ideas on the subject, I agree with Antoinette that Highbury gossip is at the root of it. Mr Elton's frequent visits to Hartfield would be enough to start the ball rolling. I rather doubt that Emma is the only person in Highbury who interests themself in possible matches for one of the town's most eligible bachelors.