Barbara Newhall Follett
en.wikipedia.org
en.wikipedia.org
Really good article from the references.
A lot of those folks were probably murdered but there's some interesting other cases of people just disappearing into nowhere. That was definitely a lot easier in a pre-Internet 24/7 connected world.
Assuming she did die in December of 1939, the Muddy River seems to be a likely place to find a body. It's just to the south of her home. The river is dotted with a series of ponds: Jamaica Pond, Wards Pond, Willow Pond and Leverett Pond. The largest, Jamaica Pond, is just over 50 feet deep. Of course, in December of 1939 they were probably frozen over.