I watched this movie when I was a kid in the 70's. Some people were in a cave, maybe scientist, maybe related family, they saw a flash a light come from outside. The vehicle did not start. They started walking. The house they came to, no one around. The clothes were laid out on the bed with a pile of ashes or dust in the middle of it. They walk into town. I think they found this boy in the pickup and a dog was crazy.