After habits get automatic you’re no longer aware of them. One day years ago a tail had picked me up when I left the house on an errand, without my knowing it, and what he learned from my movements during the next hour had cost us an extra week, and our client an extra several thousand dollars, solving a big and important case. For a couple of months after that experience I never went out on a business errand without making a point of checking my rear, and by that time it had become automatic, and I’ve done it ever since without thinking of it. That Tuesday afternoon, heading for Ninth Avenue, I suppose I glanced back when I had gone about fifty paces, since that’s the routine, but if so I saw nothing. But in another fifty paces, when I glanced back again automatically, something clicked and shot to the upper level and I was aware of it. What had caused the click was the sight of a guy some forty yards behind, headed my way, who hadn’t been there before. I stopped, turned, and stood, facing him. He hesitated, took a piece of paper from his pocket, peered at it, and started studying the fronts of houses to his right and left. Almost anything would have been better than that, even tying his shoestring, since his sudden appearance had to mean either that he had popped out of an areaway to follow me or that he had emerged from one of the houses on his own affairs; and if the latter, why stop to glom the numbers of the houses next door?
So I had a tail. But if I tackled him on the spot, with nothing but logic to go on, he would merely tell me to go soak my head. I could lead him into a situation where I would have more than logic, but that would take time, and Freyer had said the jury was out, and I was in a hurry. I decided I could spare a couple of minutes and stood and looked at him. He was middle-sized, in a tan raglan and a brown snap-brim, with a thin, narrow face and a pointed nose. At the end of the first minute he got embarrassed and mounted the stoop of the nearest house, which was the residence and office of Doc Vollmer, and pushed the button. The door was opened by Helen Grant, Doc’s secretary. He exchanged a few words with her, turned away without touching his hat, descended to the sidewalk, mounted the stoop of the house next door, and pushed the button. My two minutes were up, and anyway that was enough, so I beat it to Ninth Avenue without bothering to look back, flagged a taxi, and told the driver Centre and Pearl Streets.