I speculate that, at 4 am, everyone was asleep, except X and Murphy. (IMO dogs hear everything, he'd have heard X and the car/s and the door/s. Guessing he was also trained to accept A LOT of noise/commotion/people.)
I speculate that, because E was asleep, and everyone else, X was listening to TikTok by ear bud.
I speculate that D was awakened by some combination of doors/cars/Murphy/BK, who knows which sound worked into her slumber before it fully registered. IMO it's REALLY important to remember that, at the time, D was not TRYING to identify sounds, she's was trying to tune them OUT. She wanted to be ASLEEP. She wasn't categorizing, chronologizing, she was trying to get it to stop.
(Did we ever determine the locking mechanisms to the bedroom doors? Manual locks? Keypads? Automatic locks? Why does it matter? Implication. D, seeing BK, being scared, LOCKING HER DOOR carries an implication. Versus: D, about to lean out, was startled by a man RIGHT THERE, and closed her door, which then locked carries a different implication. And again, I think it's ESSENTIAL to stay in context: she was likely irritated by the racket and didn't expect ANYONE to be standing right there! No different IMO than if I go to leave my house and open the door, about to exit in one fluid motion, only to discover someone there, arm raised to knock. Double surprise. Just like BK would've been surprised by D if he had seen her. D is alive because she had a split second reaction time, one that helped her stop on the dime, without opening the door wider, calling out whatever she might have been planning to call out, or even vocalizing her startledness. Again, important to remember, she was startled (the same adrenaline response as shoched/frightened) to see someone who was right there, and saw that he was LEAVING-- I doubt it occurred to her even a little that he was a burglar, rapist or murderer! So many people want to ascribe blame to her for not calling 911, but I will continue to assert that D didn't know, couldn't have known that a brutal CRIME had occurred. All was quiet after that. And she fell back to sleep. No mystery. No intrigue.
I wish for myself I could pin down who said someone is here. D knows her friends, knows the layout so if she thinks K said it, maybe K did. Maybe Murphy did react to X and Doordash, maybe that's when K moved Murphy to her bedroom, maybe she looked out a window and saw one of two cars, maybe said it to Murphy or to a sleeping M, there's someone here. Not a warning as much as an explanation.
Or maybe X said it, to E. And BK heard her. I'm guessing still that X's own hearing was compromised by ear buds so, even though BK may have have heard her, she might not have heard him, st all. SHE may have NOTICED the slider being open, when it hadn't been, just moments ago.
Within about ten minutes time, D opened her door three times, only once to something of interest. It's really unbelievable (yet true) that she didn't witness more, considering what was actually happening during those compressed minutes.
FWIW I think that BK managed to overtake X before she even realized he was there. Possibly from behind, possibly with his knife to her neck. "I'm here to help you" meaning it'll be worse for you if you don't comply. Whimper.
I don't know how he encountered E but he may have had to let go of X momentarily to drop E, the bigger threat. Stepped over E to get to X who would have had nowhere to go. Those small rooms were ALL BED.
Long answer, but if I were D, I would need lifelong therapy to deal with, equally terrifying, sounds and silence. The sounds she heard, IMO nothing you'd associate with four slayings, and the silence afterwards? It was silence she sought in the first place, but not THAT KIND of silence. Dead silence.
Exactly how it sounds when everyone is asleep.
JMO