According to New York’s police commissioner, New York City had 32,000 illegal speakeasies during Prohibition. Four of them were located on the same block as police headquarters. Obviously, hiding was not the issue. Bribery was. These speakeasies were no secret to anyone. I’ve made sure that in my Roaring Twenties mysteries, the speakeasies are in plain view and operating pretty openly. I also put the occasional policeman in the speakeasy, either for a drink or to pick up his weekly “tip.”
I also make sure the plot includes a cop or two on the take. (No offense intended, Officer . . . heh, heh. . . ) One of my main characters is a young policeman who is honest, and even he turns a blind eye at the routine corruption all around him.