Behind the shocking revelations of incompetence and unprofessionalism that rocked the Secret Service this week is a longtime reporter who has been diligently uncovering the agency's secrets for years. Carol Leonnig, a Pulitzer Prize-winning investigative journalist who has worked at The Washington Post for nearly 15 years, has broken almost every single story on the agency, a series of shocking reports that on Wednesday resulted in the abrupt resignation of Secret Service Director Julia Pierson. (Pierson called the resignation “painful.”) In less than a week, Leonnig uncovered three scandals that pushed the agency’s first female leader out the door. First,...