which is in contrast to how [MENTION=42582]pemerton[/MENTION] has said he imagines wood elves as having an always-on camouflage in the proper settings, allowing them to hide because unseen
I'd prefer to say "unnoticed" - or, perhaps, "potentially unnoticed" - for the following reason.
If the elves were literally unseen that would, in the context of 5e, seem to suggest that they are invisible, and hence - even if detected - entitled to both an offensive and a defensive boost.
So I prefer to think of it this way - if A turns the corner and elf B is just hanging out in the rain/snow, A won't
notice B unless (i) B is making significant noise, or moving rapidly/erratically, or otherwise doing something that automatically vitiates an attempt to hide, or (ii) B, despite being still and quiet, nevertheless is noticed by A (mechanically, the WIS check made for A beats the DEX check made for B).
Once A notices B, though, I would say that A is not only aware of B but can
see B. Ie B ceases to be unseen. Which is to say, I envisage Mask of the Wild as closer to camouflage than invisibility.
What I've described doesn't
quite capture the flavour of the elves in the Shire whom (if I'm remembering rightly) can be heard singing but not otherwise seen or located. But it's a trade-off to stop Mask of the Wild being too strong.
Let's say a human is alone in an empty room. He wants to take the hide action so that any future passers-by in the hallways outside the empty room will not hear him. Are you saying you would not let the human hide because he's "in full view" within the empty room?
I can't answer for [MENTION=6787503]Hriston[/MENTION], but I'd be surprised if his answer is very different from the following:
The person in the room wants to be unnoticed by those passing by it, and so stands still and quietly. Mechanically, this is resolved as a DEX check for that character. Anyone passing by the room is entitled to a WIS check (active or passive as appropriate) to see if they notice the person who is trying to be as still and quite as possible. If there was a closed door to the room - so the passer-by has no chance to see the person in the room (or a shadow, etc) and is relying entirely on hearing through door/walls, I would impose disadvantage on the WIS check.