You've got to realise that he's probably not going to be cool, calm and logical when he left that house. If this is his first crime of this nature, he's likely going to be all over the shop. Even if he's a sociopath, he's going to be keyed up, full of adrenaline and buzzing with what he saw, heard, felt, and did in those frantic minutes, each killing playing over in his head as he made his escape. If what I believe about this perpetrator is correct, this attack was the culmination of years of fantasy. He'd be drunk on it, dizzy with it. He may not have realised about the sheath until he came down with a bump some significant time later, maybe when he drove out into the dark to possibly dispose of the weapon and clothing. Maybe he didn't realise it wasn't in a pocket until right then. By then, he's miles away. And his phone briefly connects to the network - I suspect because he got lost on the dark country roads and needed to reorient himself with GPS. Things haven't gone precisely to his plan. Do these small imperfections bother him? I don't know. Maybe they drive him nuts. Maybe he's so convinced of his own brilliance and godlike power over life and death that he doesn't care and doesn't think anyone in LE will be smart enough to connect those things to him. Either way, they helped to put BK into custody.
MOO