Nowhere in the novel does Brandon express concern about Willoughby, or confront him, about any possible indiscretion with Marianne.

I thought S+S3 was far too heavy-handed with its foreshadowing, making Willoughby the villain from the start. And it was kind of ridiculous, IMO, for Brandon to take Willoughby aside and interrogate him about whether or not his attentions to Marianne were honorable, long before the truth about Willoughby's seduction of Eliza II has come to light. There's no mistaking how Brandon feels in S+S3 - he is concerned specifically about Willoughby's intentions, without much cause (yet).

However, I just rewatched S+S2, and that little moment between Sir John and Brandon ("Mr. Willoughby, Lady Allen's nephew?") also makes Brandon seem wary of the fact that Willoughby is back in the country. So maybe S+S2 is not entirely innocent of trying to insert foreshadowing that is not supported by the novel. But I suppose another reading of Brandon's reaction in S+S2 is that he knows Willoughby is young, dashing, and energetic, so it may just be kind of deflating to him, and making him feel like his chances of attracting Marianne's affections just went from slim to none.