It seems from information in the The Vanished that Sandra was perhaps known for not always answering her phone. So it is possible that there are people in her life who, when they want to visit/see her, just show up. I know that my parents and their friends were this way in the pre-cell phone era. There were certain people you are close enough to, that you'd just swing by.
I agree that this is a very possible scenario. Still a bit odd though that the people who were closest to her at the time could think of no one who might be inclined to visit Sandra unannounced. Of course, such a person or persons would automatically end up becoming suspect/suspects.