Federal drug enforcement agents showed up unannounced Sunday to check at least three visiting NFL teams’ medical staffs as part of an investigation into former players’ claims that teams mishandled prescription drugs. There were no arrests, Drug Enforcement Administration spokesman Rusty Payne said Sunday. The San Francisco 49ers’ staff was checked at MetLife Stadium in East Rutherford, N.J., after they played the New York Giants. The Tampa Bay Buccaneers’ staff was checked at Baltimore …