[And here's where an actually nice person would feel bad about accidentally distressing a young lady. Sebastian is more pleased to find someone who gets it. His tone, however, is gently sympathetic:]
My apologies, my lady - had I known you were unaware, I would have phrased that differently. [Ie, he wouldn't have let it "slip" at all because then there wouldn't have been a point to it.]
As bold as it is of me to speculate on a nobleman, I do not believe he is in the habit of revealing such things no matter the person.