For those genuinely engaged in something covert, texts like this (or like this: <a href="http://www.loompanics.com/Underground_Economy/underground_book4.htm" rel="nofollow">http://www.loompanics.com/Underground_Economy/underground_bo...</a>) are for learning what <i>not</i> to do. Your enemy (whoever it may be) also knows how to read.
I'd be curious to hear any real-life stories behind slide #3 ("Patterns in shoelaces can carry messages").<p>Do they mean <i>can</i> as in theoretically? Because I could also theoretically carry a message based on myriad other things as well.<p>Seems like you'd have to get really, really close to a person to be able to read their shoelaces for the patterns in the second and third images.