inquiry agent

In Carrados’s very first exploit, “The Coin of Dionysius,” an inquiry agent named Louis Carlyle doubtfully hands him a Sicilian tetradrachm, then snidely demands of the blind man, “What do you make of it?”

For days before the ship was due to cross the Line all the officers and twenty-five per cent of the passengers became temporary inquiry agents.

I'm a private inquiry agent by profession, though this job of shadowing you has nothing to do with the office that employs me.

"That inquiry agent was a smart feller," the millionaire interrupted.

"The point is that as inquiry agents I regret to say my chiefs have decided to abandon the case, and, in my capacity as their representative, it is my duty to notify you personally."