He was very good at judging when people around him were lying. In order to satisfy his curiosity and not pique his suspicions, she knew she needed to be very careful how she answered the question. If she showed too much interest in Rosalind, Rodolphus might take it upon himself to start checking all of the potential hiding places she might have found for the pregnant witch. Maybe he'd even end up on Augustus" doorstep for a look around. She would have to tell him at least some of the truth.