Restaurants (at least in the US) have very strict standards about how long you can keep something at room temperature before you have to throw it away. Those standards are extremely conservative, and lead to a lot of food waste, but if I were on the staff I'd at least want to keep an eye on how long something has been sitting out. Those standards have just been beaten into me. You also see that on a lot of fictional…
Once you notice characters aren't eating, you'll never not see it again. Related, where they're drinking coffee from a disposable cup, you can almost always tell it's empty by how they handle it.
NCIS has a running gag about that. In the show they invariably drink... some mysterious caffeinated product, I don't know if I'd call it coffee, with a straw. It always makes a slurping sound like the cup is nearly empty. Even when just handed a fresh cup.
That show is often lampooned for the silly "two idiots one keyboard" scene, but I am convinced they are doing dumb stuff like that on purpose.