[egypturnash]

The bar example works absolutely fine for someone unfamiliar with the venue: you go to the bathroom as you normally would, and you see a sign in there that says "hey if you're in trouble do this and we will help you out", then you come back out and you do that.

There is a sweet spot between "enough people know what this signal means for it to be worth using" and "so many people know this that my adversary knows it too". The signal this kid used may be about to cross the upper threshold soon, but it worked for her.

Surreptitious and obscured communication channels have been being created and getting detected for most of human history. Look at the continuing cycle of teenage subcultures creating slang to perform the dual function of marking a member of the group and obscuring one's plans from one's parents; look at Thieves' Cant; look at histories and manuals of spycraft. And those spycraft books will probably have suggestions on what to do if you're in a situation where signals just aren't viable, too; signals are one tool.