Sometimes there are a view planes visible in a big assembly. To turn them off is kind of hard because the assy is very big and you have to check all the parts. It's kind of strange that when you open the subassy were the plane is showing, that there are now planes showed in the subassy, only in the main assy.
It would be nice if you can right click with your mouse buttom on a visible plane and hide it at once. Or is there an other easy way?