Close humans often develop certain codes in the form of body language, words, phrases or movements that let others who are aware of the code meanings to act. An example of this would be scratching your head and then coughing may mean '...he is trying to take your money'. Do you have your own code system that you use with your family or someone close? This may be very valuable in ticklish situations.
I suppose we don't have so much as a code as we have specific words and actions that are unspoken communication. We never really talked about it, but through experience together, it just kind of evolved into what you could consider code. Like when its time to go, especially if a conversation has gotten out of hand, one always asks the other when a concert starts, so the other one is able to use that to get out of the conversation. If we see someone suspicious, we always say "you dropped a penny". The other then looks for 'it' and notices the suspect or asks where the penny is. The other can then point them in the right direction so we are never blind sided.