I think he merely sloppily cleaned up the bones in his pit. Perhaps he was spooked by a family member or something.

The plate call-in: The cop was illegally searching the junkyard when he found the truck. He knew the truck and any evidence from it would be inadmissible, so they had a volunteer find it later.

I think Avery killed her somewhere on the property, stored her body in the back of her truck until the fire, burned her body. He sloppily tried to move the bones off his property.