This is all in retrospect, but if the Marquis truly did manage to narrow down the culprit beforehand, the man could've offered support behind the scenes or even tipped MC of the enemy's true identity ffs. If he was just being wary of the MC, then that wariness could have cost the lives of the MC party had they not been lucky and made preparations of their own volition beforehand.
Instead of being impressed that the Marquis showed big brain skills based on how it's written here, I just got frustrated that if that was the case, then he didn't act on that deduction at all.