Although, strangely, if you're a policeman (or policewoman, let's not be sexist about this), it's the first thing that gets mentioned when you're being introduced to someone. No one ever thinks about how that feels. You bastards.
It's just a heads up - kind of like when you take a call from your best mate while on hands free with your wife in the car. "Hi mate, how you doing - you're on hands free, say hello honey"
two of my sister's close friends are tax inspectors, that gets mentioned quite early in their introductions for the same reasons (so people don't accidentally incriminate themselves)