Greeting!  (neat site by the way).

I seem to remember in one of the books that when the main characters were doing sentry duty they had a system worked out to identify if one of their group was under duress.  I think they would identify themselves with a name starting with an A (Adam for example), to show that they were ok and alone.  They would use a B or C name to represent other things.  I can't remember how this went so could some please educate me?

Thanks in advance.