Johnny just innocently walks by his youngest son's room when he hears it. He is not at all snooping or spying on his youngest baby. He would never. He trusts his son and respects Donghyuck's privacy. Johnny will never be a over-bearing, over-protective psycho parent micro-managing his kid's every move. That's just messed up. But then again as he hears a groan of discomfort and a whiney, "it hurts" coming from inside that closed doored room all that goes out the window.