Once upon a night, Anastasia learned something from Rupert.
Apparently, Zen is on it again - he is overworking himself without telling her. It just happened that here has been some kind of problem about their means of communication with their comrades, so he was staying up late and burning himself out to solve it. With that, he doesn't get much sleep lately.
She did notice that he was dozing off from time to time, and she already suspected something. But then, he always so looks alive whenever he is spending time with her. It doesn't seem like there's anything wrong. That's why she has shrugged it off and thought that she was just assuming wrong.
"He told me not to tell you, but he didn't listen to me when I told him to rest. That's why I'm telling you now..." was what Rupert told Anastasia back then, a mischievous smile splattered across his expression all the while.