Its per 5th ed. rules (as I understand them HAHA). So if the person is out of sight, you can't see the person, etc. you can make the Stealth check. If this is in the middle of combat it is an Action. Your stealth check is vs. your target's Passive Perception. If you beat it you get to attack per Unseen Attackers. If you did not beat their Passive Perception then its just a normal attack (so the snipe moved out of sight to try and re-do stealth. He got a 12 so anyone with a passive perception above a 11 is going to see him when he pops back up).