The car being in the neighborhood does not tell me anything about the sheath. As far as I know, there is no connection whatsoever between the sheath and the car, and there is still no connection between the car, the sheath, and the murders other than being somewhat contemporaneous.
I am not pretending to be an imbecile; I am paying attention to what has been proven. Were I on the jury, the defense would not need to convince me that Furman or anybody else planted the sheath; the prosecution would need to show me evidence that the accused brought the sheath into the house and dropped it during the commission of the murders.