The first, think that this is the case of a girl who ran away and wanted to leave no trace of where she went. Whether her intent was to off herself or start anew, I think this mindset is a bit naive in that she seemed to do this without much planning, and, as all of the CJ textbooks say, its difficult enough to hide a body and evidence of a premature death. When such a premature death is your own and you are the person responsible for it, its nearly impossible. The only theory that really makes a lot of sense on this end of the spectrum is that in her haste to get away from a dui, she hid herself so well that she died from the frigid night and hasn't ever been found. With each coming year and the thaw that happens every spring, this becomes more and more unlikely.
The second, are those who think that this girl was way over her head when she left her college dorm for a few days away to clear her head. She made a multitude of bad choices in doing so (which have been gone over with a fine tooth comb here and elsewhere). With each new bad choice she was left with fewer and fewer options until she was effectively left with a.) Stick around and wait for the cops to tow her car, give her a DUI, get kicked out of school (and in her mind disappoint everyone who saw so much promise in her and be stuck working at a diner in her hometown forever more.) or b.) Get in to whoever pulled up to her's car right after the school bus driver skeptically told her "ok, I won't call the cops." and drove away. I could just see her face at this point as it occurs to her she is left with no good choices and her string of close calls is either about to end, or, she is going to escape one last time and then get her life back on track (or not), after this one last good Samaritan bails her out of the mother and father of all jams.
... Only this time, in my opinion, her luck ran out and she got in to the wrong car, with the wrong person, and has never been heard from again. I know that people are skeptical that anyone could be put under the kind of pressure to make an otherwise smart, clever, and relatively safe minded college student make such a bad decision, but my guess is that if you are skeptical of this, you haven't been to college in the past 20 or so years. The kinds of pressure a normal student has to deal with currently will make them act all kinds of funny ways, and Maura had a bad streak of choices, actions, and luck which made her act in what others would view as reckless or naive ways that are perfectly understandable to me (but would be viewed as completely unreasonable to my parents... for instance.