I don't yet have much of an opinion on the truck mentioned.
I am still working on the theory the child wandered off and got lost.
I did a "virtual fly over" the area with google satellite, and well....it's clear how challenging this search is. LE pro's know what they are doing. Here are my thoughts on how to narrow the search area....
I think the best way to find her (if she is lost) is behavioral profiling. The best source of info are the siblings, but they will have to be gently coaxed; LE who interview children know all the best approaches with dolls, games, pets, etc. Children clam up in situations like this, thinking they will get in trouble for some perceived contribution to the child's missing status, so cleverness is required.
Here is what I would want to know as a starting point:
1. Where had the child roamed off to before and was reprimanded?
2. Does she have a friend near by (even older) that she would want to visit? (Get cookies from elderly neighbor, etc.)
3. Did she leave anything at school/pre-school she would want to retrieve or go look at?
4. Was there a store near by where she would want to go buy candy or treats?
5. Animals near by that she wanted to go see? I noticed there is a hay ride place fairly close by. Did she like going there to see the horses?
6. Where did she like to hide?
7. etc. etc. etc.
Most important, her siblings likely know what mischief she liked to get in to. They need to be queried again. That territory is really challenging. IF you've ever participated in a search, you know how hard it is to find people outdoors.
Amateur opinion and speculation