HONG KONG—A number of investigation firms are retreating from Hong Kong and moving staffers out of the city, the latest sign of concern among foreign companies as business dwindles and Chinese authorities crack down on corporate intelligence gathering.

Three U.S. and British due-diligence companies that established a presence in the city more than a decade ago have exited or are winding down their operations this year, according to people familiar with the matter. They include Nardello, a global company built by a former U.S. federal prosecutor, and the Risk Advisory Group, co-founded by a former British fraud investigator.