[sirtrex]

Whenever a dungeon boss is about 5 minutes from leaving, a system message posts indicating such

For instance:
 "Another rumbling sound is heard, something large is on the move"

Once the boss has, left another message plays
"The rumbling has ceased, whatever it is is gone"

Running to the end of a dungeon just to find monster tracks can be pretty time consuming at times and disheartening. Getting some indication that the boss is leaving, or has left can save players alot of time while grinding. 

The warning message will encourage players that want to boss hunt to hurry to there at flank speed. The left message will save players time, and a trip to the back of the dungeon if they don't or can't make it in time.

Bosses make  a large noises when they enter a dungeon, so their sudden absense should leave some impact on the environment ICly as well.