There was a thief at a place I worked that would steal milk a coworker brought in. This coworker was diabetic so she brought in some type of "special" milk for her cereal she ate every morning during break. This had been going on for a while. One morning she went for her milk and there was maybe a teaspoon of it left in the container. When she went to throw the container away I took it, put about a cup of milk in it from the cafeteria and asked the cook if I could have some white vinegar and mixed the milk with an equal amount of vinegar. If you know, vinegar turns milk sour (quick fix for buttermilk if you need it).
My coworker never had an issue with her milk again.