"They all looked at one another. An illegal would mean big trouble. Since the beginning of espionage, a foreign spy with civilian cover, posing as a native-born citizen of the host country with a meticulously prepared legend, speaking fluent, colloquial language, and living an unremarkable life with a humdrum job, had been the perfect faceless solution to handle a sensitive asset in enemy territory. No official status. No diplomatic installation. No intelligence-service connection. No profile for the mole hunters to search for. And everyone in the room knew the Russians prepared and deployed illegals better than anyone."
Palace Of Treason, by Jason Matthews (copyright 2015; quote is from page 334 of the Pocket Books/Simon & Schuster Inc. mass market paperback edition, July 2016).