[She notices the slip-up of course, and she is rather certain she knows the name as well. All the same... she continues without faltering, offering him a gentle smile.]

My servant had left me just the same as Sebastian left... Master Ciel. [When speaking of topics like this... she doesn't wish to continue with the same level of formality.] However he returned some time after, a bit worse for wear admittedly, but with all his memories intact.

So the possibility should not be disregarded, all the same.