The fact that the car was found close to home, and the couple were last accounted for at home, suggests to me that something happened at home, they were taken in their car to a dusty location, and the person that is responsible drove back home. If a total stranger was responsible, the car would not be close to home. I think it would be in the dusty location with the couple.
How old are the children?
For what it's worth.......
The Nancy Grace transcripts...
Now, on the facebook page for them, the administrator posted awhile ago, that there was Dust on the car, not mud.
The transcripts to Nancy Grace mention Mud.
Also the transcripts mention that not only the car doors were unlocked, the House was unlocked as well.
http://transcripts.cnn.com/TRANSCRIPTS/1506/24/ng.01.html
"GRACE: Whoa! The car was unlocked, too, Valerie?
PARASO: Yes, that`s what friends of the family have said, that the car was unlocked...
GRACE: OK, that is all wrong.
Marc Klaas, who`s going to leave your car on the side of the road unlocked and your home unlocked? You know, they have five children. Three
of them were in the house, it`s my understanding. Why would you leave your home unlocked with your three most valuable treasures in there, your
children? Uh-uh!
KLAAS: Nancy, I completely agree. It does seem highly unlikely. You know, Nancy...
GRACE: Something ain`t right.
KLAAS: You`re exactly right. In fact, this has shades of the McStay case, I think. I mean, it`s people who disappear without a trace. It`s a
vehicle that`s found in a separate location. It`s in the desert. This is a very troubling situation."