No documented proof but from I've heard it's small trades that signal other MM's of certain situations. It has nothing to do with the September 11, 2001 incident.
911 Trade = an MM ordering Pizza, if the trade is followed with 100 it means I'm in for a slice, if a 200 follows the 911 it means order two I'm hungry. 300 means three toppings and 500 calls for a Pizza Party I'll bring the beer.