I thought maybe some of the other AE speakers would want to comment, because I don't think the implied context, whatever it might be, is familiar to me. But for what it's worth ...
The only context I associate with 'officer' is police. If I had to guess, I would say that the car rental clerk (?) is making the assumption, whether serious or humorous, that the German man is a police officer, or some other type of law enforcement, but in civilian clothes. Maybe he has an upright bearing, or a brisk walk, or a short haircut, or a discreet earphone with a wire? Or maybe his passport or ID simply reveals some law enforcement credential or title, like, indeed, Agent, Detective, etc.?
If you were actually referring to the series with Bryan Cranston, I don't see where a German character comes into it.
In any case, I would say that it's probably not really a car rental office, and that you probably need to give better context if you want better answers.