In the movie "Casino," a scene early on showed the Mob's "skimmer" simply opening the door to the "counting room" of the Vegas casino with his own key. He simply walked over to a steel cabinet where stacks of money that had been counted were sitting, took a number of stacks, put them in his briefcase, then flew back to Kansas City. The counters ignored him.
The physical act of "skiming" in a casino has to be done before any money is recorded in the casino's books.