I'm pretty sure they mean checking if she's left, not if she's there. As far as I can tell, no, there is no way to tell if Bonnie has left yet besides seeing her on a camera feed, which is annoying.
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True, you can't see if Bonnie has left without seeing her on cams, or checking the door.
Normally, I would just turn on the light beforehand, quickly open the door and see if she's there, and if she is, then I'd close it just as fast. But I guess that's too much of a hassle, especially for harder nights.
there's a slight shadow at the door every time bonnie appears, similar to the regular game, if you want to learn how to discern an occupied door from an unoccupied one id memorize the look of the hallways when bonnie isn't there, then turn on the lights look at it again when she is, you'll notice the difference pretty quickly. so now you dont have to use the camera or risk her attacking you by quickly opening the door