When you think about it, EVERY PRE-prepared food you buy, like Stouffer's Mystery Meat patties in gravy in your frozen food's department, is LEFTOVERS from Stouffers. So some of our food supply (if you buy that stuff) is already leftovers when you buy it.
I read a story (a long time ago) that when the companies that make cake mix first came out, the cake mixes were made with everything but water. What they found was that housewives couldn't take 'credit' for making the cake if they only added water. They needed to have some ownership in the making process. So the cake mix company left out some ingredients (eggs, oil) which gave the housewives some ownership in the assembly of it. That way, the housewife could say SHE made the cake. Essentially, because the housewife added some new ingredients. When is a piece of cake considered a leftover?
When is something a leftover? It appears that leftovers can go through the magical process of having something new added to them or heated, and they become new food. Another magical transformation of new food into leftover food, is food that is transported from one venue to another (served at the birthday party at the office, then brought home).
I think the word leftovers has a fuzzy definition, often subjective, defined by each individual.