Each creature must find the hidden one
For example, there's a sniper hidden in this photo. Try to spot them.
So, your instincts are right, some observers may be able to see the hidden creature and others may not. If those that do tell those who don't where to look that would probably give them advantage. For passive Wisdom (Perception) that's a +5 - that ...
It is not possible.
As you note, the rules for attacking while hidden state that "If you are hidden when you make an attack, you give away your location when the attack hits or misses." This is fairly conclusive, but maybe you have found an exception;
Using Umbral Sight has the following effect: "While in darkness, you are invisible to any creature that ...
No. Boots of Elvenkind produce no sound, but they do produce vibrations
A monster with tremorsense can detect and pinpoint the origin of vibrations within a specific radius, provided that the monster and the source of the vibrations are in contact with the same ground or substance.
It detects vibration through the ground or other ...
When you are hiding, you are trying to hide. You are not necessarily unseen, nor unheard, and your location could be known.
When you hide your character tries to hide in the normal sense. Your character physically does something to hide themselves. They crouch behind a box, slow their breathing, stop their armor from rustling, etc.
If you make noise, your ...
One red herring to watch out for is being 'behind' someone. In combat everyone is constantly moving around, they have no facing. Out of combat you can certainly hit someone from behind though.
The monk does not have enough actions to do this, but there are situations where it will work.
For example a Rogue can do this:
Attack from melee range
Use Cunning ...