When Jane Kennedy and DS Higgins enter the farm, DS Higgins goes up some steps and tries to open a door. He remarks that the door is shut but you can see the hook that shuts the door is on the outside. He could have gone inside simply by unhooking the door.
The cop and coroner's assistant are sitting in the Combie van which won't start. They are contemplating a walk back to town. Then the cop looks in the side mirror and sees a car approaching on the road. Then a long shot shows the Combie is parked perpendicular to the road. There is no way he could have seen the road in the side mirror.