Okay, I'll bite.
Bears son from a previous marriage did this when he was about 6 - 8. He has developmental problems, so mentally he was younger. He also had disconnect issues with other parent.
I treated it as a need for affection/ food for the heart. Food can have a soothing affect, and Mo may be sneaking food as a way to self comfort.
He certainly can't express or even understand what is happening, but if he were able to he might explain to you that he knows food is available and that he won't get in trouble for eating since you are the one putting it out for him. He might also tell you that he knows that he can ask for a snack and you will provide one, and probably one he would like.
Then he might, if he could, tell you that sneaking the food is different. It isn't the food, it's what it stands for. It isn't the food, it's the "hunt."
In other words, if he "pretends" the food is forbidden, and he can get away with sneaking it, he can in affect provide for himself (hunt down) the affection / love that he feels is forbidden or that is not available to him. The food is simply a substitute.
Deep stuff, I know.
I tried reasoning with step-son, explaining that he couldn't keep food in his room cause bugs would find it. Tried to make a room full of bugs sound disgusting, but he liked bugs! I tried offering him food throughout the day.
Finally, I tried catching him in the act, distracting him and sitting down to read or watch Power Rangers with him, or play checkers. Somewhere in all of it I would work in a few comments about how we all have to be able to trust each other and how "sneaking" hurts that trust and how would he feel if I snuck into his room and took things. Over time he stopped. I don't know that it was due to anything we did at our house. He may have just mentally worked through it.