At P.J. Clarke's, the iconic bar in the east 50s in Manhattan, once you walk past the main bar into the initial dining area, there are two hidden alcoves, one to the right and one to the left. There's a table and a few chairs inside each, obscured from view. I've mentioned these rooms before as perfect niches for clandestine meetings or trysts.
What I hadn't noticed before was a little speakeasy-like, peek-a-boo window connecting the southern room to the the barroom. Perfect for a patron beckoning a waitress or bartender's attention, or for a waitress to pass through a plate or drink, or for a fella to keep an eye out for any arriving enemies.