Inside a cab, the "blind" thug strikes the back of Chuck's head. Chuck was told to drive down a back alley, but the background screen continues to show them moving in traffic when Chuck is (presumably) knocked unconscious while driving.
McCormick doesn't pay Slip for the cab ride to the airport. It escapes everyone's notice.
As the Bowery Boys force their way into Mr. McCormick's mansion, a hand from outside can be seen closing the front door behind them.