I've seen it before. It's a signal from a waitress to one of the customers. It means: Meet me in the parking lot in one hour. Bring cash. :smile:
I think you're on to something here, Tramp.
Years ago, in the Canadian Maritimes, if a man drained his beer glass and placed it on the table upside down....it signalled that he would fight anybody in the house.
So all those times I spent wondering who gambled in this firetrap.....
...the upside down chalk meant they would play anybody in the room.
Who knew?
I used to think it was Murphy's Law.....
...if you drop a slice of bread, it'll land butter side down...
...if you put a piece of chalk on the rail...yep, blue side down.

Even critters can't escape Murphy's Law