In the part where Adam escapes from Saturn, Barbara shows him the ceramic figure, and it is floating. Yet, later in the film, Barbara is holding a knife and the dad doesn't notice it. Don't you think he would of noticed a floating knife?
I've wondered that a lot too. When Adam's headless body runs to lock the attic door, would the living see him bump into the railing (which gives a bit when he hits)? There were a few things it seemed the people would notice.
I think that the reason that they dont notice stuff like that is because they are so stupid. I mean, have you seen Delias art work? No wonder it tries to kill her because it is mad at her for not being pretty art that people actually want to look at.