He might have found her remark amusing enough to laugh, but Hermione wasn't completely joking. Several minutes later when he was dressed for dinner, Antonin promised her at the door of their bedroom that he would keep a close eye on the girl to see if he noticed the same odd behavior. Considering he was an excellent judge of character, except when it came to the women he allowed in his bed, she felt comforted by his insight. Maybe she was just being paranoid.