Lyxen
Great Old One
The Devs specifically mention you keep the benefit of being hidden for the attack so they might see you, but you still get advantage.
It's not exactly what they said, compare to the exact rules:
- If you are hidden — both unseen and unheard — when you make an attack...
- If you come out of hiding and approach a creature, it usually sees you.
Now, if firing from the undergrowth, where you can be hidden and not move at all to attack, I would, as a DM, be more generous. All of this is 100% RAW, the rules are fuzzy on purpose so that they can be interpreted by the DM according to the circumstances.
Being able to assume one's location doesn't make it less unseen and unheard (hidden). You might have a good idea where it is, but not when it will attack and from where exactly in 5 feet about.
Yes, but as a DM, I want to reward players who think like their characters and project themselves in the game world. So if a character does nothing with regards to the rogue, I might give the latter advantage, but if a player says "I'm very wary of the rogue, I try to keep track of him", I will certainly take that into account as well. Call it a flourish if you will (and therefore allowed even by the strictest of RAW combat rules), but the rules actually encourage that as well.
And, I hasten to say, both ways, it might be easier for another threat to surprise the player above, because he is focussing more on the rogue. Thanks to the fuzziness of the rules, you get much more interesting combat, more roleplaying and intrigue, because the first rogue might just be a distraction, and so on and so forth.
A player actually lost a duel that way in a campaign. It was Amber DRPG, but the principle was the same, the player kept looking to the audience of the duel to see how they were reacting to the situation and the insults, and although he was the superior fighter, he still lost the duel. It's all in the narrative, and the good thing is that 5e supports it RAW out of the box, if you use all the mechanics.