My own guess is that she's not in Hanson. I think she was transported somewhere else, voluntarily or otherwise. Sandra could have walked away from her home, but she surely would have been noticed by someone in that particular neighborhood on a warm summer late afternoon/evening while it was still light out, and I don't think she'd have gotten far wandering around in the woods after dark.
I understand your point about locating bodies, but if she fell, or had a stroke, or intentionally harmed herself her body likely would have been in an easily accessible area. As for water, given where she grew up and the fact that she worked on a commercial fishing boat with her husband, I'm sure she could swim if she fell in a pond or lake. No gators in MA. At least not yet!
With that said, I think it's also worth remembering that no one reported hearing or seeing a car picking her up.