Since you're still on induction, definitely avoid the popcorn. Being allowed 20 grams of carb per day doesn't mean that you can have them all at one time; that defeats the purpose of limiting your carbs which is to avoid causing an insulin reaction. Eating 20-25 grams of carb at one time will cause that in most people, especially if they are high glycemic carbs like popcorn.
My concern with the movie theatre butter wouldn't be hidden carbs, but hydrogenated and transfats. No matter how much they claim that stuff tastes like butter, it's definitely not butter and is full of the bad fats.
If you have to snack while watching a movie, bring your own treats and take a wide berth around the snack counter.
Hey! You'll save money that way too.