When I think of those 3 things missing, I get one image in my head. She was brushing her teeth (either remembered and got back up after everybody was in bed, or got up earlier than the parents and did it then) and someone threw the comforter over her to help absorb sound so nobody woke up. Unfortunately, that in no way explains who it was or how he/she got into the place.
My thought process is more along the lines of it being a Saturday morning and Saturday mornings with kids means one thing, great cartoons early that you can only catch on Saturday morning.
Perhaps she woke up early, dragged her comforter downstairs with her to sit in front of the tv and watch her cartoons.
Now this part may be reaching for it, and of course lots of factors, but I can completely envision a child not wanting to miss what is on tv and proceeding to run and grab toothpaste and a toothbrush then running back in the living room and brushing there.
Someone gazes in the window, knocks, she opens the door and they lure her out, seizing the opportunity.
Side note - I used to always find my toothbrush wet because my daughter wanted to use mommy's toothbrush, or if we were using similar ones because she would forget what color she had. She never was in to getting character toothbrushes and bubblegum flavored toothpaste.
Also, regarding mom and step dad not hearing, lots of potential factors as well. If they live in a neighborhood that never sleeps, they probably have created their own white noise for the bedroom, whether that is loud fans or simply a loud television. I have an uncle who used to sleep with his volume on the tv 3/4 of the way maxed out.
I honestly believe mom and step dad have nothing to do with it. If anything, mom may appear nervous or stuttering on her words simply due to the recent custody situation (which is still yet to be clearly defined), just getting AN back and somehow AN managing to disappear on her watch, so to speak.
Finally, for the record, hearing the pronunciation of AN's name, I think the correct spelling must be Ahlittia.