 Why was Humphrey being observed?
What made it possible was Humphrey Fownes' abstraction; he was an uncommonly preoccupied individual. In the ensuing contretemps they emptied his rear pants pockets, got his wristwatch and restored the contents of the handkerchief pocket. It all went off very smoothly, like a game of put and take—the sole difference being that Humphrey Fownes had no idea he was playing. Humphrey Fownes strode through the puffs of falling glass still intrigued by a temperature that was always 59 degrees, by a humidity that was always 47%, by weather that was always Optimum. It was this rather than skill that enabled the police to maintain such a tight surveillance on him, a surveillance that went to the extent of getting his fingerprints off the postman's bag, and which photographed, X-rayed and chemically analyzed the contents of his pockets before returning them. Sitting behind the wheel of the orange car, he watched Humphrey Fownes approach with a distinct feeling of admiration, although it was an odd, objective kind of admiration, clinical in nature. Lanfierre's job was to ferret out aberration. Lanfierre was almost proud of Humphrey Fownes. He had even contrived to engage him in conversation once, a pleasantly absurd, irrational little chat that titillated him for weeks. You've got winds in your brain, Lanfierre, breezes of fatigue, zephyrs of irrationality—" At that moment, all at once, every last window in the house slammed shut. It expressed his weariness and distaste for people who went off and got neurotic when they could be enjoying a happy, normal existence.
 he didn't act like he was expected to
 he didn't act like he was expected to  to make sure he wasn't a danger to society  he was to be observed before he was allowed to be married  he was suspected of committing crimes