I would post this in the special needs forum OP. But a few things - any history of trauma, adoption etc? Either way, just generally I would recommend trying to take the power away from this interaction by 1. don't ask her any questions you already know the answer to. She will probably not tell the truth the first time and then that just leads to more frustration for both of you. She lies because she feels shame. Shame can make us do all sorts of things so I would try to address that - does she have a therapist? 2. Can you try a snack basket of things you all pick out together that she has access to at any time? Try grocery shopping together and coming up with a plan for snacks together.
Also, I would try talking with her about it, asking if she's hungry? Since the hiding and sneaking tells you she feels ashamed, try connecting with her when this happens. "Were you hungry last night? Maybe we need to think about snacks that will fill you up more." Is it possibles she is having snacks at those times but they aren't particularly filling? It's so hard to know from your post because it could be related to the ADHD, or if there is past trauma it could be related to that, but otherwise sneaking and seeking "fast" carbs often means our body isn't getting what we need, so it seeks the fastest way to get energy - simple carbs. So to rectify this you could try more filling snacks - snacks that include a carb, protein, veggie/fruit. Maybe a banana peanut butter smoothie for the bedtime snack with plenty of good stuff in it, something that will really meet her nutritional needs.
Also I'm not sure you're going to get very well informed answers here to be honest, so I would tread lightly. If you're worried about weight and that's why you're limiting snacking you could try the book "How to help without hurting" by Ellyn Satter. If you think it's trauma, adoption or even ADHD related - you could try the book Beyond Logic, Consequences and Control. It's geared towards kids with attachment difficulties but there is a chapter on sneaking food that might have helpful ideas.