"had observed that" is a correct and usable part of a sentence in written English.
You can use it to talk about something that happened in the past. Example: He had observed that his neighbor always left for work early in the morning.
In Mr. Carlin's class one morning, Kim Bruno, 22, a biology major who graduated from the State University at Binghamton in December, expressed concerns about a science class she had observed that morning.
She had observed that her mother had begun to shed a few.
Simon had observed that "willies" have been looming increasingly large in Bennett's work.
Davydova, it read, had observed that the base had emptied out.
Pasteur had observed that, besides filtration and the application of heat, exposure to certain chemicals could eliminate germs.
In the past, Dr. Gentner and her colleagues had observed that children who home-sign did not appear to invent gestures for locations spontaneously.
Earlier, he had observed that traits one could accept in a novel's protagonist, or in a complicated friend, often seemed loathsome to modern moviegoers.
Epidemiologists also had observed that in America during the war years, when meat and dairy products were strictly rationed, the rate of heart disease temporarily plummeted.
Blaustein, who was studying the mating behavior of frogs and toads in the Cascade Mountains, had observed that some long-standing populations simply weren't there anymore.
He had observed that the use of force and the threat of prison seemed to have little effect in deterring gang members' behavior.
Women as early as the 16th century had observed that cows that consumed ergot miscarried their calves.
