This question suddenly came to me today. It seems like a silly, trivial question, but I find it a very difficult one to answer.
Imagine you would be working in a hospital, and you would be responsible for distributing the patients' breakfast and for collecting the dishes afterwards. In this breakfast, a few slices of cheese or ham are included. Imagine that a patient doesn't eat all of his or her slices, and some of it is returned to you in the kitchen. What would you do with the remaining slices? Would you hand it out to the next patient, and risk spreading infection? Throw it away, and waste food? Eat it yourself to avoid either?
Imagine you would not be working in the kitchen of a hospital, where strict hygienic regulations are in order, but in that of a restaurant, or a boyscout campout, or in an industrial kitchen? What would you do then?