as such, “I would tell on them, and possibly anonymously.” I think they are gauging your loyalty by asking this question. Some people might actually answer by saying, “well, it would depend on what they were doing,” or “it depends on who it is.”
But the company does not want you making these decisions for them. Theft is theft, no matter if it’s petty office supplies or something of greater value. Theft cannot be tolerated in the workplace and violators must go. Though I would not rant on like this in your answer, just be brief and firm.