Empty or Not?

I went to put plastic wrap over a bowl of left-overs, and this was all that was left in the box. Isn't there some kind of unwritten rule about not leaving insufficient amounts? A dribble in the milk jug, two small broken pieces of what was once the bar of soap in the shower, two squares of toilet paper, or 1 1/2 inches of the plastic wrap is close enough to empty to warrant NOT PUTTING BACK for use later.
You know who you are.