Eun-Huai knew from the beginning that DOG was in the secret room?

Yes, at first, Yang Mo was allowed to hide in the closet of Xu's bedroom, but he accidentally discovered the secret passage connecting the home and Xu's bedroom. But Yang Mo met the one hiding his father's body on the way, and later he was able to control Yang Mo, so Yang Mo hid in the secret passage connecting Yuan's home from the beginning.