dnd 5e – If a creature detects a hidden creature, do all other creatures also automatically detect you?

Each creature must find the hidden one.

For example, there is a sniper hidden in this photo. Try to detect them.

Hidden sniper

I've seen them, I suppose not, they are at the base of the third tree from the front to the left, just above the trunk that lies on the ground. Can you see them now?

So, your instincts are correct, some observers can see the hidden creature and others cannot. If those who say to those who do not know where to look, that would probably give them an advantage. For passive Wisdom (Perception) that is +5, that may be enough to allow some who did not see them now to see them. Otherwise, they can use their action to search with advantage.

Of course, if the sniper "came out of hiding," everyone would see them.