Leo is the Soviet agent who is the protagonist in this series, and he's pondering why he had assigned men to surveil a suspected spy who subsequently fled Moscow rather than simply arrest him:
Leo was to blame. He'd given this man, a stranger, the benefit of the doubt. He'd presumed he was innocent; the kind of mistake a novice might make.
Better to let ten innocent men suffer than one spy escape.
He'd disregarded a fundamental principle of their work: the presumption of guilt. ... Those who appear the most trustworthy deserve the most suspicion. Leo recognized it as a play on Stalin's well-known aphorism:
Trust but Check.
Stalin's words had been interpreted as:
Check on Those we Trust.
The duty of an investigator was to scratch away at innocence until guilt was uncovered. If no guilt was uncovered then they hadn't scratched deep enough.Updated March 22, 2014: A fan of Darkness at Noon quoted one of the two passages I remembered most. The other passage I remember is how the state would deny the guilty the heroism of expressions of dignity before a firing squad by marching the condemned down a long underground passageway. At an undisclosed, varying point in this march, the executioner, trailing behind, would execute the condemned with a bullet to the back of the skull. Eeeek!