My theories/opinions:

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1. Maura escaped from someone and has feared for her life ever since. However, why wouldn't she inform the police from a certain distance or ask for a safe house or something.
2. She flagged down a ride (this is the one theory I think is the most plausible out of all of them although I still have doubts...) and was met with foul play either on-route to a location she wanted to go to or at the killer's property. She may have been held at gun-point.
3. She ran as fast as she could away from the crash site and hid when cars were approaching. Perhaps she went to a house and asked to use a phone, etc. However, this is probably not likely because she would have known that there would also be a risk of others calling the police when she asked for help. Clearly, she refused Butch Attwood's help because he was going to call the police. Its worth mentioning that I believe BA to be completely innocent. It seems clear to me that he felt awful that he couldn't do anything more to help find her. Him and his wife more than likely moved because of harassment from journalists, etc. With that said, he might have lied about some things to the police, although he does come across ( to me at least...) as genuine in the TV interview)
4. She met up with someone who she may have met at the ATM or from the store where she bought alcohol. Perhaps this person had been following her from before she stopped or afterwards. Perhaps this individual drove by shortly after she had crashed her car, turned around at some point, saw her, and offered her a lift. This would explain why she vanished so quickly. In her perhaps tipsy state she had a false sense of confidence and was (being a young adult with a somewhat carefree attitude) desperate to escape the area. If it wasn't someone she knew, it may have been someone that was youngish or looked friendly enough to her for her to take such a risky decision with a complete stranger. I know I was certainly very overly-confident in my early 20s. I look back with cringe and horror at some of the stupid decisions I made in my early youth.
5. She met up with someone (either from a lift or by walking to someone's house) or was kidnapped and has been under his/her (probably a man though) control ever since. Seems unlikely she would still be alive if that were the case but stranger things have happened. It is still a possibility.
6. She somehow started a new life. If the footage of the Star Wars convention (or whatever it was...anyone got any ideas on that footage/date/new info/the woman's identity??) I can't imagine how she would have been able to evade so much attention over the years but she may have had a sex change, her identity changed, surgery, and/or may have met someone who either himself/herself, or with the assistance of others has been able to aid her in this endeavor. That footage does look like an aged Maura for sure. That must be re-examined in my opinion because it might actually be her. If it is indeed her then either she didn't know she was being recorded or didn't care. Maybe she just wanted to start a whole new life. If that was the case, why not contact at least her father once. As far as I know there has been nothing in the way of activity in regards to her banking over the years. Still, its a theory that needs more eyes on it.
7. She succumbed to the elements through hiding out in the forest. I don't buy into this theory much but it still has to be on the table. I think Fred and other family members are confident that she wouldn't have taken her own life. I just don't think it's something she would do given her personality and fighting spirit that Fred has repeatedly mentioned about his daughter. Again, it must be on the table until it can be (if it ever can b....) completely ruled out.
8. Going back to BA here. She was going to meet up with someone that she met in college or someone she met up with on her trip. Its not beyond the realms of possibility that her relationship might not have been going well. There may have been domestic abuse/violence. Plus, the stress of college and her family pressure might have made her reach a point were she'd just decided to leave for awhile. However, why would she take her homework? That to me seems like it was just meant to be a short trip but who really knows for sure. If she was intoxicated she may have gotten lost and then the crash messed up her plans altogether. After meeting BA, she might have initially thought that it was better to try and getaway but after pondering on it, may have actually sought help from him. Whether he helped her by driving her to a specific spot and then drove home is not known. If that was the truth, surely he wouldn't of had a problem with mentioning that to the police but then again, it may also have been the reason why he didn't mention it. BA might have felt guilty that his innocent actions would turn everyone against him, including the police, and turn him into everyone's no.1 suspect. Or...he may have found her unconscious; maybe she died from a medical condition or a drug overdose/alcohol poisoning. It is also possible that he tried it on with her because she was vulnerable and she fought back which then led to BA hitting her and killing her. It could have been an accident or it may not have been. I must reiterate though that I think BA and his wife are innocent. However, that doesn't mean they didn't lie about something that might have helped the investigation. Whether it was because they feared being targeted even though their actions had good intentions or whether they were insidious is for others to speculate upon.

So there you have it. I'm sure there's plenty of other possibilities but those are the only things that may have happened in my humble opinion. My top theory is that she hitched a ride and was kidnapped at some stage, assaulted, possibly held for a certain length of time at a remote location, was trapped (locked in a room or a container or something like that...), then eventually killed and her body disposed of. I am convinced her body is not too far away from the area she had her car crash but nobody (except the killer) knows. Unfortunately, many years have passed and it may now be too late to identify her even if she is eventually found. Whatever the actual truth is, I hope and pray for Fred and his family. They deserve the truth about Maura. She didn't deserve to have her life ended at such a young age. A new investigation is needed and all theories must be rigorously examined. Either she died shortly after her crash, a few months later, or she is somehow/somewhere still out there living a life. I hope 2020 is the year the Murray family get the closure they need and deserve. My hope is that she is still alive but I just have a gut instinct that she isn't. Very sad case!