Uber Technologies Inc. is among a growing number of United States employers who have added background checks for existing workers to the existing practice for pre-employment screening. According to Scott Hall, chairman and chief operating officer of the National Association of Professional Background Screeners, membership in the organization more than quadrupled last year. The Chicago public school system has responded to a Chicago Tribute expose on sexual abuse in schools by rechecking the backgrounds of 45,000 employees and thousands more vendors and volunteers, but employment lawyers and worker advocates warn HR managers to be vigilant of erroneous information.