Immediately after the statement, "I cannot," he sniffed the air and said, "You use Evian skin cream, and sometimes you wear L'Air du Temps, …but not today." This is to imply that he is that perceptive and can accurately detect scents. This is meant to unsettle her. So it is very possible he could tell the difference between fresh and coagulated blood. Or it could be a reference to her menstruation. I think it is intentionally left vague to show that, despite his confinement, she is vulnerable to him.