I, of course wonder if he went back to where he left Susan's body- to make sure it was not visible (from the top of a cliff, or something), or to make sure it was still covered, and that animals had not gotten to it.
But, I also wonder the opposite- that Josh may have thought that the rental car did have GPS on it, or that he was being followed, so he drove around aimlessly, just to throw off the cops. Driving around for hours just to confuse the authorities, seems like something he would do, in my opinion.