...or olfactory or whatever other means of perception they might possess... Whether or not it makes sense is a different question altogether. But my point was mostly the first paragraph: as long as you are not hidden, people know your position. That is the (only) difference between hidden and invisible/heavily obscured.
...And yet I try to make sense of a badly written paragraph which is both incompressible from a RAW standpoint as well as from a RAI one. This IS the question.
Although as a DM, in conditions where hearing does not apply for reasons mentioned before, and the player is unseen in some way, I would - given that the targets do not have other senses - probably make the stealth check an auto-succeed. Whether they would be considered hidden before the check is up to you, but RAW they wouldn't be. In such cases we are talking about -10 to passive perception. If you run with passive stealth or something like it, then yes: People are hidden by default in such conditions.
"unseen in some way" ?????? You can't get more vague than that..
Boy, I would not want to play a rogue in your campaign where everyone can hide in the market for free... But we are dangerously treading into homebrew which does not help the discussion...
To answer the question you started the topic with, however: Normally you have to be "unseen (behind something, obscured, invisible, or otherwise out of sight)" which is only the case with heavily obscured, because you are not "unseen" in lightly obscured.
... How exactly do you go from unseen to heavily
obscured? I do not want your interpretation or "inner feeling" please. Just quote me the text from the book.
Just like 'behind something' means something that provides total cover, not just a lamp post.
woaahh ... That's reaching.
The ex-Skulker feat changes this part of the requirement to only require lightly obscured. So still no lamp posts, but "a moderately wooded area, lingering smoke from a sacked town, dim lighting" all will do just fine.
Again, do not ask of me to deduce a core mechanic out of an optional feat. This is simply wrong.
I will give you this: The wording could be upgraded to "unseen (behind something that provides total cover, heavily obscured, invisible, or otherwise out of sight)" and while that would not change the rules it would be more clear indeed.
oh... Not only does this change the rules. It flips them on their head!
For what's its worth, I will give you my interpretation of what is really going on ...
At some point WotC realised how poorly written Stealth was, by which some people were preposterously claiming what you are saying: That you need to be heavily obscured in order attempt a hide check. So what did they do? They took the phrase
You can't hide from a creature that can see you.
and via an errata, they turned it into the infamous:
You can't hide from a creature that can see you clearly.
... Moreover, in order to wash their hands ala Pontius Pilate, the added the all inclusive:
The DM decides when circumstances are appropriate for hiding.
To the crashing majority of players out there it was clear that the intention all along was to say without saying that you can now hide while lightly obscured
(which makes perfect sense just like it made perfect sense in 3.5 ... and Pathfinder if I am not mistaken). As you can imagine, the fact that "Skulker" & "Mask of the Wild" remained unchanged spurred even more chaos in the forums... Understandably they could not rewrite the entire handbook. The wanted to make a significant change ... without actually making it ... with minimum effort and without turning the tables. They wanted to keep everyone satisfied and even-more-vagueness was the way to go.
Unfortunately this change was not accepted in cheers ... Players need rulings and not "The DM knows"
for the 100th time.
Level Up did the same mistake. They too chickened out of an explicit ruling.
... which ofcourse makes you wonder why, since they were free of previous mistakes...
When you specify lightly obscured
and heavily obscured
and yet you employ neither of them for a core mechanic such as stealth that desperately needs one ... it cannot be by chance ... not after how 5e treated the matter.
If I was to guess, they were either sloppy or ... most likely ... they intentionally took the middle/vague road which did not have the risk of alienating a portion of the fanbase.
Excuse my english. I am not a native speaker