Bonny is visible without having to open the door through her shadow! its just that the shadow is really hard to see...
Thing is, there is a small graffiti of an alien right where the window is, and if bonnie is on the other side if the door, her shadow blocks the alien graffiti!
Another thing that i noted was that the footstep sound queue whenever bonnie or chica get to your door has 3 total footsteps before the sound ends. If you wait for the third and final footstep to completely end, you can close the door, wait 2 to 3 seconds (i do 3 seconds just in case) and that will guarantee they will have left the door by then, meaning you wont have to waste the extra energy needed to turn on the light.
hope this helps.