Nothing to say he hadn't been in that house previously either. While everyone slept. He might have had firsthand knowledge of the fishbowl bedrooms, the unlocked slider, the living area step.
I don't recall what time he made his loops that night but ostensibly he was waited for lights out, wanted to enter when everyone was asleep.
IMO he was unaware of the DoorDash delivery, missed it by minutes, or he might have made one more loop.
It's possible he'd been in MM's bedroom on other occasions, maybe not stealing things but moving them. That's so creepy. Vague violation... you'd feel like something was off, you just wouldn't know what... but could well be how he learned her name, if he did know it.
If, if, if, if KG hadn't come back that weekend, the household may have gone quiet sooner, BK wouldn't have had to loop, might have slinked in unnoticed, commit a singular murder (with SA overtones) and linked back out still unnoticed. One victim, less the bloody massacre he left behind, fewer sounds, no Murphy, maybe no voices. No witnesses. No height, no build, no eyebrows, no DNA.
All that would remain, an indistinct white sedan, easily with a stolen license plate.
In some ways, it was with her very life, KG laid hers down to identify MM's murderer (and hers), for I think it's due to her presence, BK lost control, leaving the sheath behind.
He might have gotten away with murder otherwise. Four of them.
JMO