I think if your mother was murdered, yes, it could be that her killer did something like that. Close by, as you describe, or farther away, just depending on a lot of factors. It sure would have been good if some searches of that area had been done back then... but I guess LE wasn't looking in that direction, apparently.
That is one reason, of course, that it was so important for her info to get into NamUs, in case she is deceased and her remains should be found somewhere -- and sure hoping LE (law enforcement) is working together with you/your family to get the DNA samples going, so she could be matched if that should happen. I know that working toward that has to be a grim reminder for you and your grandmother and probably for your sisters, too (even if they "seem" disinterested) about the loss of your mom from your lives and the terrible shadow of what may have happened to her. But then, too, I feel, from what you have posted, Lataviahughes, that if she should be found deceased, you would want to know and be able to bring her home, back to her family.
Did her boyfriend claim that he last saw her at their home? (I'm thinking he did...?)
Wish we could do more, Lataviahughes.
Are you getting anywhere with some of the other stuff you have mentioned ... getting your siblings interested, talking to the guy who was in the hospital a while back, etc.?