I think BK knew his vehicle would be captured on some cameras close to the crime scene; there just wasn't anyway around it. To counter, he chose a route that misdirected south, eventually into rural areas without camera coverage. He waited in this area for a time before eventually traveling east and then back north headed for Pullman. With his phone turned off, his calculation was that LE would only have the color, make, and model of a car that was produced in the hundreds of thousands, that traveled in a direction away from his residence. In his mind, this was acceptable. That was his plan.
IMO, BK never expected LE to pull camera footage in Pullman (kudos to LE!), and this is what eventually put him on their radar.
I think BK also took precautions not to leave DNA, and I won't be surprised if DNA on the sheath is the only DNA of his found at the scene. IMO he put the sheath in a sweatshirt/coat pocket after entering the residence and it fell out in the physical violence of the assault in Maddie's room. In the dark or dimly lit room, he never saw it, and with an adrenaline flushed brain he probably didn't even think about the sheath until he got south of Moscow. I don't believe the sheath was left intentionally... I think he returned to the scene later that morning in hopes that the sheath would be found where he had parked.