While in possession of Runaan's Hurricane, you gain a small floating orb that accompanies you around the battlefield. This orb fires up to two additional missiles, functioning similar to Lulu'sPix, Faerie Companion.
The passive will be triggered by the auto-attacks of any champions classified as ranged. This is an attribute applied to champions that is independent of in-game context, such as proximity to the target or a specific champion's attack animation.
Shapeshifting champions will trigger the effect while in ranged form only, as they are classified as melee while in melee form and the orb disappears returning when they switch again.
Rengar'sUnseen Predator will not trigger the effect despite his autoattacks having 600/750 range, as Rengar is classified as melee.
Kayle will trigger the effect while Righteous Fury is active despite her attacks having no projectile themselves.
Azir will trigger the effect on both his standard attacks and his command soldier attacks, permitted his target is within his personal attack range (i.e. the item will not trigger while attacking targets using his soldier's extended range).
For effects such as Azir's soldier attacks and Twitch during Rat-Ta-Tat-Tat, the Runaan Bolts will remain single-targeted. The bolts can potentially damage targets already damaged by the abilities.
Although Gnar is officially melee, and simply is given some extra range by his passive, Hurricane works when he is in Mini Gnar mode, and Ravenous Hydra works while he's Mega Gnar.
Runaan's Hurricane can be affected by blind, as such the secondary bolts will also deal no damage.
Twitch will still trigger the effect while Rat-Ta-Tat-Tat is active. The two missiles fired are unaffected by the ability, and remain single target, targeted missiles. As such, it is possible for an enemy to be damaged by both Twitch's pass-through attacks and this item.
Attacking turrets will not trigger the effect. 
The following items trigger when damaged by the secondary bolts: