I believe that the knife sheath was carelessly left behind by BK BUT another scenario could be that the killer purposely left a particular type of knife sheath at the scene to throw LE off… so they would be looking specifically for a Kabar knife when in fact a similar but different type of knife was actually used. That gives the perp time to clean and dispose of the murder weapon or even hide it to retrieve later as a trophy.
BK may have been posting on SM and discussed the knife sheath having been left at the scene. Or LE or someone else close to the investigation could have leaked some details to a trusted source who then posted it on SM or perhaps even one of the friends who came to the house in the morning opened M’s bedroom door and saw the knife sheath lying on the bed. So there are others who could have posted this accurate information before it was publicly revealed. IMO, BK’s arrogance would support him posting on SM. All speculation.
Another thing I’ve pondered, if BK had a bag or back pack with him, he probably put the knife in It before leaving. And he knew the knife sheath wasn’t with him when he left. With barking dog and other noises from the victims, he knew he needed to get out of the house quickly. He didn’t have time to look for the sheath and probably would have to turn on a light on to find it unless he lad a head lamp on so he opted to leave without it. IF he saw DM looking out of her bedroom door in the dark, and the knife was packed up, he could have made a snap decision to leave the house without attacking her in hopes that she didn’t get a good look at him.
Also, I thinks it’s remarkable that DM had such a good description of BK considering it was dark and she saw him for a matter of seconds. I’d agree with bushy eyebrows! Also, I think it was wise to document that she woke up “about 4AM” because if she said a more specific time such as 4:02 or 4:05, she could be held to that exact time in court. Now there is some wiggle room.