Very simple. KGB had network of informants, your best friend could have been one, every workplace supposed to have one. And that was very good network of very f

Very simple. KGB had network of informants, your best friend could have been one, every workplace supposed to have one. And that was very good network of very frightened people, so generally nobody knew who was in there. Now decades later, if some informant is uncovered, the reaction usually is - never would have guessed.