A.A.
If you know she's snuck something because you see the evidence, don't ask if she's had it, you know the answer and are just setting her up to lie. Ask why she ate whatever it was upstairs instead of at the table. Try to learn why she's sneaking food, tell her what items are ok for snacks and let her have as much as these healthy options as she wants in the kitchen. Just try not to make an environment where she feels she's going to get in trouble or shamed for eating when she's hungry. You say she's predisposed for a weight problem, I'm assuming you or/and dad have weight issues? If she has seen adults in her life struggling with food issues it's likely she is applying that to her own eating habits.