Actually, they are flagged in the system as 'hidden' so while they are dead and unable to be targetted... they are still on the map.
The code that determines when a battler exists is in
Game_Battler:
#--------------------------------------------------------------------------
# * Decide Existance
#--------------------------------------------------------------------------
def exist?
return (not @hidden and (@hp > 0 or @immortal))
end
This is used for determining when a character is dead (along with the def dead? routine) and is used for enemy targetting purposes. Right now, can't have one without the other... for now.
Hope this helps.