The only other option that is somewhat viable in my mind is some sort of horrible accident in which case she should have been found nearby. Maybe she went out to smoke a cigarette and hurt herself or had some sort of medical emergency.
I've ruled this one out because no body has ever been found. Hanson was scoured and unless a body was very carefully weighted down in water, it almost certainly would have turned up with so many around looking for her. There's a good amount of water around in Hanson, but it's all lakes, and bogs, and ponds,
Robbery is always a possibility, but robbers don't normally take residents with them**. Having seen the neighborhood, I can't imagine a thief randomly picking Sandra's little house. The street is quite literally off the beaten path and densely populated.
As for abduction, why? And why no signs of struggle?
If the door was unlocked, there is a plausible scenario. My understanding is that SIL took care of all communications with the mechanic that day, and I'm not even sure she went inside with him. If she handed over her key chain with her house keys on it, she may have figured she wouldn't need them as she'd be home until the car was ready, presumably on Friday, if not even Thursday some time. If something came up and she decided to go out, I can see her leaving one door unlocked. I've done it before myself when my husband accidently took my house keys with him and I needed to go out. Locked the front door, and left the back door unlocked. If like me, she didn't have any jewels or cash in the house, she may not have been too concerned, especially if she planned on being back later that night.
I too am very curious any phone calls and what LE knows about them. Sandra didn't disappear from home, IMO. If she did make it home that day, she left her house, probably late Wednesday afternoon or evening. My guess is that she left voluntarily as any scuffle would have been overheard by neighbors. We know she didn't have a car, so she either left on foot or in someone else's vehicle. Unless she went to see a neighbor, on foot isn't too plausible given the limitations of the area. So by process of elimination, I think she left with someone voluntarily. Maybe for a cup of coffee and a quick chat about something? Or out for a few drinks after a long day? Something happened when she was with this person or these persons and that something is what led to her disappearance.
Beyond this - who, why, where, how - I hope law enforcement has some clues because I sure don't.
**I just saw the Tara Grinstead 48 Hours episode, and it sounds like the motive was probably not robbery. Both perps knew the former beauty queen and it's believed one or more likely both entered the house when she'd gone to bed with the intent to sexually assault her. She was killed and her body cremated in a pecan grove over a period of days to hide forensic evidence of the assault and murder. While it's possible something like this happened to Sandra, we still have the issue of the closeness of multiple neighbors. And disposing of a body in that manner would not be plausible on the South Shore of Massachusetts, although a burial in deep woods somewhere is always possible. With some seriously good advance planning.
Sandra's house also had all new windows and exterior doors and central A/C. If someone came in that night, she probably let them in as a locked up house would have been a major chore to get into.