I walk behind a pillar while you are watching me. You can't see me and I can't see you. I am not hidden so you can hear me. You know where I am
I roll a stealth, beat your passive perception and hide. You still can't see me and I can't see you. You can't hear me either as I am hidden. You know where I am.
I move out to attack you. Maybe I 'peak', maybe I 'lean', maybe I 'step' - it doesn't really matter since you know where I am and you are possibly still looking in my direction. I am no longer hidden since I need total cover to hide and I no longer have it. I now attack
DM 1 rules that you didn't notice me step out. I get advantage on my attack roll
DM 2 rules that that you were still watching the pillar, see me step out and I get no advantage
DM 3 rules that while leaning out I am still hidden without cover and I get advantage even if you are looking at the pillar
Both DM's 1 and 2 are correct by the rules. I don't believe DM 3 is. I am more like DM 2... I think you are more like DM 1 if I understand your argument. That's why I think we both generally agree but differ on how easy this is to achieve in combat.
A rogue could do this every round with their bonus hide
I pretty much agree with this. I would rule as DM 1, unless I had a good reason to rule as DM 2. Hiding several turns i a row would be one, so would a creature spending an action to search, but failing, so would readying an action to attack if you appeared.
I know but I think the movement is an essential part to the hiding when trying to make use of just cover. Being heavily obscured is a different argument because your opponent typically can't see you while you can see them. You don't even need to hide
I get where your coming from, but I'm not sure that that's entirely accurate. I think the *possibility* of movement is important. If I hide behind a pillar, you have a pretty good idea of where I am, but you can't be positive, because I
could be somewhere else. Perhaps I teleported, or there was a secret door behind the pillar, or I climbed up it. Because I'm hidden, you don't know for sure if I'm still there or not. You have to check. If I wasn't hidden, you would know if I did any of those things, because you could hear me.
Granted, you could (correctly) assume that I can't do any of those things, so you can be pretty confident of where I am, but you have no direct evidence of me being there because I'm hidden. (i.e. you couldn't prove to anyone else that I was there who didn't also see me hide there). And in that situation, if you were watching for me, I certainly don't think I would get advantage on an attack.
Mike Mearls was referring to an example of a Halfling hiding behind a party member by using the hide skill and the obscurement rules not total cover. The Halfling can see the opponent, the opponent cannot see them. The Halfling does not need to step out, move or in any way reveal themselves to shoot. They simply shoot from where they are with no chance for the enemy to see them assuming they hid well.
True, but I think that the situation applies to a creature hiding behind a tree round after round. The only difference being that in the pillar situation, the rogue would need to make herself visible
before the attack and not a consequence of it.