He likely heard voices downstairs on the main floor. And kept his knife out on the way down from MM's room. And after killing Xana and Ethan it was probably time to put the knife away and leave... he probably noticed the sheath was gone. JMOYeah, I totally agree. You don’t take tremendous efforts to leave no DNA, and bring none home with you, yet leave an object behind that could reasonably contain your DNA.
He takes it off, begins stabbing, and gets caught up in the moment. I’d have killed to see his reaction whenever he realized he left it behind.
I bet the guy did the math...and with the loud noises, screams, barking dogs, he decided not to look for it in the dark rooms and hallways and decided to leave.
MOO