The most effective way I know to sneak a snack without anyone knowing is to hide it in my bag and sneak it up to my room with a distraction "Hey, look at what the cats doing!", something like that! Otherwise I use hide it with my body, always keep the other person looking elsewhere or concentrating on something else rather than what is in your hand! Or cover it with something, another plate to keep it warm if it's a cooked food, a slice of bread, a cloth! There are many sneaky ways to go about snacking without a soul noticing... But sneaking them to my room and hiding packaged snacks is the best way I know!