Fighters in hiding
The attacking fighter cannot target an enemy if the enemy is both Prone (either Pinned or Seriously Injured) and in partial or full cover — they are assumed to be keeping their head very low!
Oh, ok. 'Hidden' is also a condition that is used with the Pitch Black rules. While it also results in the fighter being untargetable, it's completely separate from the 'Fighter in hiding' rule. Rules for that can be found page 120 of the rulebook.
A fighter with the Hidden condition who makes a ranged attack loses it for the rest of the round unless the weapon has the Silent trait (and Gunshroud is one way to make a weapon Silent).
And the two can apply at the same time, so a fighter that is Hidden (condition) but Hiding in cover (Prone behind cover), could make a blind fire action, becoming revealed (unless using silent weapon). Regardless of Hidden/Revealed, the fighter will remain Hiding in cover so can't be targeted.