They are ghosts...
”The "ghosts" Special Agent Pappas refers to here are the four bank robbers known as the "Ex-Presidents." He calls them "ghosts" because they have been so successful at eluding capture and committing their crimes without leaving any useful clues. It is amusingly ironic that the leader of these "ghost"-like bank robbers is played by Patrick Swayze, who played an actual ghost in the movie he made right before this one: "Ghost" (1990).”