I've heard that a few times, that there were no footprints. For how many miles was this? What if she ran a couple miles to get away from the scene and then crossed a driveway or ditch to get into the woods, old cabin etc. ? So there were no footprints from the road up even a driveway for a couple of miles? She had every reason to flee the accident. The Disappeared show mentioned a coke can that smelled like alcohol, a box red wine in the backseat and some pink liquid that had been poured in the snow beside the car. Since she was still on probation, she had very good reason to get away as fast as she could from the car wreck. I think she died from exposure however there might be a slim chance she took a ride with the wrong person who may or may not have stuffed the rag in her muffler. This rag is the one bit of evidence that doesn't fit the first scenario.
I really think the rag is a non issue, and likely something Fred, may have told her to do in case of emergencies or whatever, and not realizing it serves no purpose. I pretty much agree with everything else.
If she had been drinking then it would be logical that she would want to get away rather than face the consequences of a dwi. If that is the case I doubt she would take her time walking away and would more likely run. She wouldn't have had much, she was a distance runner, likely running on the road or to the side of the road where the snow wouldn't have been deep. I don't believe they used a blood hound, even though that's what they say in the disappeared episode. I believe they used a dog that tracks by air scent rather than ground scent. An air scent is easier lost than a ground scent, and blood hounds have been known to follow scents even when driving away in a car. I think she could of made it further down the road then what they thought. Also, assuming she had been drinking the alcohol would of made her more susceptible to hypothermia, and once that sets in logic can become severely impaired.
An argument could be made that she hitched a ride with someone asking them to take her someplace where she could get cell phone reception, and was abducted.
To me those are the only two likely scenarios. In either case the motive would be getaway from the scene, and avoid getting a dwi. She sobers up and explains away the alcohol opened in the crash.