From a technical stand point, when AI is idle it's simply waiting for some trigger to change it's state.  Waiting for the player to come into view or make a noise.  From a philosophical stand point I like to compare AI to the light in a refrigerator.  When the door is closed we assume the light is off.  So when we can't see AI we assume they are standing idle.  But what if...

Ladies and gentlemen I present to you the first of what will hopefully become a frequent and welcome occurrence here at GI online.

Idle

 - All sprites were obtained from http://spritedatabase.net