My kids are too young to really be snooping around in the fridge but when I was younger the rule was "If you see it and want it, it's yours". Funny enough they never ate anything of mine unless I offered. I guess they wanted to make sure I had first picks.
So how is it in your house? Let's say, for example, you bring home leftovers from a restaurant. Is anyone allowed to just grab that box when they get hungry? Or if you stop at the store and buy just 2 kiwis. Is it understood that those are yours or is the house free to take one?