Most SUVs have dark tinted windows in the rear passenger and cargo area. Someone could have slipped into the rear seat and hid, then surprised GP with a demand when GP got behind the wheel. The neighbor may have missed this, because the perp was careful. The Palmgren house is set back from the street, which would have made it hard for anyone to see someone slipping into the car at certain angles.
I think the neighbor waved at GP while GP drove out of her driveway, but the neighbor got no response back. My impression is that the neighbor was walking to get mail or was out for an exercise walk. I could be wrong. The phone calls may blow my theory. It is an area I need to do more research. Something happened to GP. It just doesn't make sense she would leave her children.
Someone could have slipped into her vehicle in the Walmart lot as well, or anywhere else she stopped for that matter. I wonder if she had the habit of locking her vehicle's doors?