Think about. Look at Fred.
He can never see light that originates from outside his plane. The lightbulb in Bob's room for example, may shine light onto Fred's world. A lot of this light goes right through Fred. Maybe it Bob's wall and bounces back out, so Fred's insides are not only visible but completely illuminated to Bob.
However, there are light particles that fall directly onto the 2D surface of Fred's world. But, for Fred to see them, they would have to bounce off the object into his eyes. However, they come off at an angle equal to that they came into his world with. In other words, they actually come into his world for a brief moment and then bounce right back out. Every part of his world is completely illuminated with 3D light that he doesn't even notice!
The same may be happening to Bob. Every area in his world may be illuminated with 4D light that he can't notice. But his insides and the insides of everthing around him are completely visible AND bright to Emily. Areas that are dark to Bob are constantly being struck by 4D photons. This gives a crazy perspective on spying on lower dimensions.