I am still reading up on this case. The SUV going down the embankment backwards with out Bryce in it suggests two options:
1) He pushed it over.
2) Someone else pushed it over.
If is was accidental, he would have been in it and left some evidence of his egress from the vehicle.
And with all due respect to a previous post, there are always foot prints or "sign" no matter what the ground conditions. I have tracked across concrete, carpet, tile and macadam. In cases where there are no clear prints, there are still impressions or push downs in the vegetation, broken twigs etc... And unless Bryce got into another vehicle, somewhere along the upper road, there are signs of his passage....at least there were.
FWIW... I was in my car once when my husband got out to check something out and apparently forgot to put on the parking brake. I decided to get out too and it rolled about 5' before I could jump back in and stop it. Mine's an automatic. Husband's car is a stick shift and he didn't have it in gear once - it rolled out to the end of our driveway and nearly into the street, which is maybe 30-40' (actually probably longer than that.. I've never measured it). I don't know exactly what Bryce's car was like, but if he's in the habit of jumping out without putting on the parking brake, I can see his car rolling off the hill accidentally.
I do agree there should be some kind of footprints, though. Even if they're not Bryce's, it's hard to believe that whoever did this left zero footprints.