I just wanted to add something, in response to the question of why anyone would suspect foul play:
I think that foul play is the most likely scenario for a few reasons. The first set of reasons involve a lack of clear premeditation, as outlined well by Captain Kahn, above. I don't see why she would pack up her belongings, not wait for/stock up on cash (instead of using her card), make calls, only pack enough for a day or two, etc.
The second set of reasons, and the most compelling in my mind, is the lack of evidence. It's not easy to just pick up and start a new life from scratch, to disappear. If she had been near the Mexican border, maybe. But it's hard to convince people to give you a job, a place to stay, a way to keep your cash, all without having an established credit history. My friends who are newly established citizens in the U.S. have a heckuva job just trying to get simple services, and they have their paperwork in order and aren't hiding from anyone. Even if you find some nice old folks who live on a farm and make an arrangement, someone in town would see you. Her case may not have been a big media event, but it has been on public television for years. People are recognized that way all the time. And even if she could get a decent fake ID that will get her over the Canadian border (also not easy), the public infrastructure (healthcare, job welfare, low income housing, etc) is all based on citizenship, which she would need to prove somehow.
If she were alive and living somewhere else, there would probably be evidence of some kind. Sightings, at least. Maybe she would use her name for things. In many other cases people continue to use their real name as they make new friends and word eventually gets out. The hypothesis best served by a lack of evidence is foul play (by which I mean abduction or an accident where someone hit her in the dark and then disposed of the body). You aren't going to find evidence of foul play by and large, because it's strongly in the best interest of a specific party for you not to find that evidence. And people will go a long way to prevent that evidence from coming to light. If she had died of exposure, they would have located her body quickly and nearby, or Fred would have found it during one of his many searches. Foul play, while grim, is the most parsimonious answer.
I also just wanted to state that my previous comment was not a statement intended to reflect on Maura's intelligence or capabilities. I believe that she is probably a very smart, savvy person. But she was experiencing a series of trials at the time, her judgment was compromised by emotion and drink, and she was by accounts experiencing some trauma. I think that speaks to a state of mind that doesn't support sound judgment. And I do believe that continuing to go to her father (going to his hotel room at night, etc) supports the idea that she was actively allowing him to help her sort some of her problems. Certainly she did not, for example, have much in the way of clothing with her. She wasn't even dressed for the weather, which shows poor planning in general. And if she were really taking off, why wouldn't her boxes of possessions be with her? It is exactly because I think she is intelligent and capable that I don't buy the idea that she was intending to take off and make a new life.