There is no Hidden Club in 5e
Hidden Club was invented to explain the tightly-interlocking mechanics that determined whether a D&D 4e character had the Hidden status. The very first premise of 4e's Rules of Hidden Club is the all-caps and repeated statement that everyone always knows where everyone else is. It's repeated and text-shouted because it's completely non-intuitive when you're coming to 4e fresh.
That's not the case with 5e. Coming from 4e it may seem self-evident that there must be a Hidden Club equivalent in 5e, but there isn't. That doesn't meant that there aren't rules, just that nothing like 4e's Hidden Club is necessary to explicate hiding in 5e.
How hiding works in 5e
The rules for hiding are deceptively simple. Deceptive, because if you expect them to be more complex, it's easy to unconsciously add in the expected complexity. But they really are quite simple:
- If someone can see you, you can't attempt to hide from them.
- Someone doesn't see you when they're not looking at you.
- Once hidden, you can be detected via sight, hearing, or both, so don't be seen or heard.
- You can hide in plain sight, given a moment of being unseen or unattended and an appropriate concealment strategy
These don't even really need numbered list items, because they're exactly how hiding works intuitively outside of RPGs. It's a convenient breakdown for the following rules cite sections though. Once we get through the citations that support those four points, we can dispose of them.
Interestingly, none of these depend on whether you're in combat or not, as we'll see.
1. If someone can see you, you can't attempt to hide from them.
This is the simplest cite. D&D Basic Rules v0.1, page 60 (sidebar):
You can’t hide from a creature that can see you
2. Someone doesn't see you when they're not looking at you.
In combat, most creatures stay alert for signs of danger
all around, so if you come out of hiding and approach
a creature, it usually sees you.
A creature needs to "stay alert for signs of danger all around" in order for it to only usually see you. The contrapositive's implication is that a creature who is not alert for danger all around does not necessarily see you, and whether they do or not is a matter of DM judgement of the situation. The rules-as-written for how creatures can visually detect enemies is therefore exactly the same as how real-life vision works. (Adding in fantasy forms of vision is a trivial exercise that doesn't alter how this rules text reads.)
Consequently, although this bit of rules text calls out combat in particular, the guidance it provides for how hiding works is not limited to combat. Combat is just a good example of a situation in which "most" creatures are "usually" on high alert and hard to escape the notice of.
Further evidence, from the same sidebar again:
However, under certain
circumstances, the Dungeon Master might allow you to stay
hidden as you approach a creature that is distracted, allowing
you to gain advantage on an attack before you are seen.
A distracted creature as described is one not alert to danger coming from your particular approach, even if they're otherwise alert to danger. This dovetails with the intuition that the rules simply describe how sight, focused attention, and inattention work in reality.
3. Once hidden, you can be detected via sight, hearing, or both, so don't be seen or heard.
If you deliberately make noise or move from your hidden position, you are revealed (to those who are paying attention, as above). This is automatic (q.v.):
make noise […], you give away your position
if you come out of hiding and approach
a creature, it usually sees you
(The "usually" in there refers to the stipulations above, as previously established.)
But you can also be discovered simply by not being as quiet or as cleverly-concealed as you think you are (q.v.):
When you hide, there’s a chance
someone will notice you even if they aren’t searching. To
determine whether such a creature notices you, the DM
compares your Dexterity (Stealth) check with that creature’s
passive Wisdom (Perception) score
So if you do a so-so job of hiding your position or you're not staying absolutely silent, an opponent with keener senses than your ability at stealth will know you're there.
4. You can hide in plain sight, given a moment of being unseen or unattended and an appropriate concealment strategy
If you hide yourself before anyone knows you're there, your hiding spot can be in plain sight if your Stealth roll is just that good. Similarly, you can use a moment of distraction to conceal yourself, so that the opponent doesn't know where you hid when the distraction lets them attend to your general location again.
This is a pretty cool implication of the relatively simple rules. It's also still entirely in-line with the idea that hiding works just how it intuitively would in reality.
The party is advancing along a curving corridor when they hear footsteps and voices approaching. The party hasn't tried to be stealthy, and they hear the voices rise in obvious alarm at the party's audible approach.
But there is still a moment before the two parties round the bend and spot each other, and the halfling thief throws herself to the side of the corridor where the floor meets the wall and in the same motion nimbly drapes her cloak over herself, completely concealing herself. She appears to be an innocuous pile of rags in the corner when the orcs finally spot the party.
If the thief's Dexterity (Stealth) roll beats the passive perception of the orcs, they won't notice that the bundle of rags is in fact a bundle of stabby sneak-attacking knives until it's too late — or until they actively investigate the bundle, which they're not doing because they're busy actively engaging in combat with the visible threats. The thief had better take advantage of her hiding spot before her friends are all slain and the orcs can investigate her hiding spot!
This kind of thing nicely matches how Arnold's mud-covering trick works in the first Predator movie, as well as Peeta Mellark's riverbed hiding spot in The Hunger Games. And it's mechanically simple to pull off in 5e with core rules, with no need for Feats like Hide in Plain Sight to be allowed to attempt it. You just need to set up a situation where you can try to hide in plain sight, and beat the Passive Perception of your anticipated opponents.
Of course being hidden by being in total cover or darkness is even better, but it's not the only way to be hidden, and that's pretty awesome for stealthy characters.
So hiding is fairly intuitive: create or find a situation in which you'll either be overlooked by observers (harder to pull off, but common), or blocked entirely from their sight (easier to pull off, but needs suitable objects), and don't make any noise that they might hear. Then, hope that you're more skilled at stealth than they are skilled at noticing hidden things, and that further they won't come looking for you and actively test their perceptions against your skill.
Appendix: But hiding isn't all there is to Stealth
It must be emphasised that Dexterity (Stealth) covers more than just hiding — hiding is just one stealthy application. You can approach someone from behind quietly (q.v.), you can creep through shadows in a location completely visible to someone so long as they're not given reason to suspect a danger in your location and and you don't draw their attention (again: move quietly, move carefully). You can use Stealth to slip away from a dinner party without being noticed. You can use it to creep along a ceiling beam above some alert guards without making them look up. Stealth is much more than hiding.