So once all subterfuges have been published, no subterfuges will work any more?
In a world with a finite number of spies, most spies are not being watched by other spies at most times. And spies assigned to watch a particular target spy are generally less highly trained than the target. Simple tricks to avoid attracting attention from casual observers are still useful.
> Simple tricks to avoid attracting attention from casual observers are still useful.
Correct.
And methods such as molding cocaine into faux PVC pipes are not, precisely because they are interesting/memorable and end up documented in customs agents' handbooks.
My shoelaces cross over each other four times in each shoe. Using the different directions of crossing, that's eight undetectable bits. You can do a lot with eight bits.
But read subltle differences in shoe-lace patterns after a glance and cross-reference with the list of meanings? Hard. They'd just look like a mess to me. I suspect that the most you can get out of it reliably is about 2 bits, so four signals, including one zero state meaning "no message".