You can see when Bonnie has left the door. When she's there you can just see the dark hallway. But when she isn't there, you can actually spot a Bonnie plush in the bottom right corner of the glass. That's how I tell if she's there.
Okay so I was wrong. It isn't a Bonnie plush. I'm not quite sure what it is to be honest and I feel dumb for not getting the reference. Nonetheless there is still a figure of some sort at the bottom right of the glass when Bonnie isn't there.
