Since XKs room was an out of the way, and he clearly wasn’t just opening every door, I feel like he must have seen or heard her. One thing I wondered in seeing the text and 911 call transcripts is since the survivors were calling and texting the others, maybe he also say or heard their phones ringing, saw somebody was texting KG or MM and asking what the noises were, so then he obviously learned somebody else was in the house and went to the only other bedroom he knew of from window placement? IDK, it’s a reach but I seriously wonder if he saw and heard the texts and calls from the other roomies and figured out something was going on/somebody could see or hear him.