Kind of orthogonal to the story, but anyone who steals lunch or food from a co-worker should be fired immediately. There's a 0% chance that someone who steals others' stuff is a good, honest person in other parts of their life.
Mistakes happen. There are all sorts of ways you could accidentally eat someone else's lunch, and having a 'zero tolerance' policy is a good way to end up firing an innocent person.
Want to give us some examples?