Why would BR have had to take KRs car out on the 29th when he could have used his own car, unless the fuel gauge bumping story is true (which I doubt).
It is possible that KRs car left on the 28th and was brought back on the 29th, perhaps by BR.
If there was a dead body in a car cadaver dogs would pick up the scent Id think, unless investigators are withholding information.
So probably a planned abduction occurred first and BR then picked up the car and brought it back home. Hence KRs phone ping in Gisborne and BRs in Diggers Rest.