@MonaLeeza,
Also, many of these grocery stores I shop have a cafe area where you can sit and eat. I've also bought drinks there as well or snacks, paid for it at that registered and walked around the store with that food.
When they were younger, I've even brought food into the store in baggies.
Are all those acceptable?
And in the cases of when I've been to stores that do not have the scanner, and upon checking out, I've given the half drunk bottle of water or whatever item to the cashier, they never seem to have an issue with it. They simply scan it in, smile and hand it back.