To gain sight of; to discover at a distance, or in a state of concealment; to espy; to see.
To discover by close search or examination.
To explore; to view; inspect; and examine secretly, as a country; -- usually with out.
To search narrowly; to scrutinize.
One who keeps a constant watch of the conduct of others.
A person sent secretly into an enemy's camp, territory, or fortifications, to inspect his works, ascertain his strength, movements, or designs, and to communicate such intelligence to the proper officer.
The great advantage of being a writer is that you can spy on people. You're there, listening to every word, but part of you is observing. Everything is useful to a writer, you see - every scrap, even the longest and most boring of luncheon parties. Graham Greene
It's part of a writer's profession, as it's part of a spy's profession, to prey on the community to which he's attached, to take away information - often in secret - and to translate that into intelligence for his masters, whether it's his readership or his spy masters. And I think that both professions are perhaps rather lonely. John le Carre