So this is a standalone version of the very cool Maskar's Oblivion Overhaul feature that makes actors able to react to missed arrows/spells.
Enemies will start to search for shooter, friendlies will flee or complain - there's variety of scenarios that make the game feel more alive.

Besides ripping it off from the MOO 4.9.4.2 I've also done some minor changes:

1. Fixed any projectile counting as an arrow in terms of detection distance - spell projectiles will now use the correct ini setting.
2. Added an option to limit the max amount of projectiles being tracked at any given moment, defaults at 5. If there's more it will stop tracking of the oldest projectile.
3. Limited the max distance between the shooter and the projectile to 8000 game units to prevent some arrow projectiles falling through LOD and being infinitely tracked until cell change.
4. The chance for enemy to immediately start combat with the shooter will only roll if the shooter is in the enemy's LOS and at close enough distance (less than 2000 game units).
5. Added a few more debug console print events.

After that, all credit goes to Maskar.