In my current fortress, a giant bat decided to go on a rampage through my dining room/meeting hall.
Nobody seemed to care unless they were being actively targeted by the bat. The bat would be hacking away at a cow or a dog, and dwarves and animals would sit there or go about their business normally. The same happened when the bat decided to attack a dwarf. Other dwarves just a couple of squares away didn't seem to notice.
One hauler even walked over the bat while the bat was actively attacking a dwarf...
Even the military didn't seem to care. I immediately put my military on duty and brought them into the dining room... and they just sat there while the bat made dwarf and dog hamburgers. I set their squad setting to "Squad harasses dangerous wild animals"... and they still sat there. I stationed a squad right on top of the now-exhausted giant bat... and they moved up next to it, but still ignored it! The bat attacked one of the two squad members; the combat lasted a while, but the other squad member, despite being 2 tiles away, did nothing.
Then the dwarves threw a party in the carnage while the giant bat decided to wander around in the nobles' quarters. Unfortunately, no nobles were harmed.
(I finally got the bat to target one of my legendary+97891 miners temporarily drafted as heavy militia. Once the dwarf was being targeted he duly attacked the bat, as normal. His squad-mates still stood around and watched the combat from 2-3 tiles away rather than joining in, however...)