Has happened to us about 4 times. I left one truck untouched for about a week and upon checking on it found it almost completely absorbed into a large pile of boulders, barely visible. Same with the pic I'm uploading -- one truck eventually merged with the creek itself. If any of us get in the driver seat, we disappear and die and then the truck also finally despawns. Don't know if it then respawns at original location or elsewhere, but this could be a problem with the 15-truck limit.