Sometimes things occur to you when you least expect them! There are some interesting parallels here....
Col Brandon and Willoughby have both made rather hasty exits from the Barton scene and both under circumstances which are less than clear.
Col Brandon says he is called away on business, but seems rather upset - cue Mrs Jennings' suspicions that it concerns his illigitimate daughter
Willoughby says he is sent away on business by Mrs Smith, and also seems rather distressed. Elinor hardly knows what to suspect.
Col Brandon does not know when it will be in his power to return.
Willoughby doesn't seem to have a declared intention of coming back any time soon....
Following my theme of deception, there are certainly some secrets here. Elinor is trying not to think W guilty of deception, but wonders at his behaviour. and what sort of deception are W and Marianne practicing? their behaviour says they are engaged, but they've not told Mrs D and Elinor positively, which with Elinor counts against it being a fact.