Not sure, but OSHA will have a problem with an imporerly labeled container.
Edit, thinking about it. You probably would have a solid case - especially if there are damages. Some else said it’s like putting sprem in food, but I think blood would be a better comparison [citation defintaly needed].
The theif could simply be allergic to something in her milk that’s not in the creamer. If the woman, unknowingly or not, had any sickness that can spread via breast milk, her leaving that in the creamer container where anyone could drink it is very bad. It could spread from the “theif” to their spouce, who spreads it to the mail man.
Don’t leave breast milk in a unlabeled container, public fridge or not.