"A private investigator?"

There's some slight surprise in the way he echoes those words back at the other. It's a very uncommon thing in Billy's time, but it does exist.. Even though the instances of those aren't usually the most legit businesses.

In other words, it has the unfortunate implication that makes Billy wonder if he's maybe talking to some sort of criminal who has slightly changed tracks. Enough so for him to ask: "What sort of matters did you investigate?"