Back when I worked in corporate America, I never left items in the employee break room. Call me an obsessive compulsive paranoid weirdo, but leaving opened food or drink in a dirty fridge where people I either didn't know or didn't like could access it always seemed a bit weird to me.
According to this article on KOCO, it looks like my behavior has been validated. Some employees at an Oklahoma clinic caught a janitor doing inappropriate things to their peanut butter: