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I ruled that if you are invisible, for example inside the darkness spell, enemies have disadvantage on attack rolls on you and enemies have to guess your location, how to deal with this second part?
Do enemies have to use action/bonus action to make a check? What if they simply try to walk inside the invisible creature space instead, do they stop before enter in his space and automatically know his position?
Invisibility only provides that:
For the purpose of hiding, the creature is heavily obscured. The creature's location can be detected by any noise it makes or any tracks it leaves. (PHB 291)
Thus they still have to hide to make their location unknown to others, but they can do it anywhere. This is a Dexterity(Stealth) check versus the Passive Perception score of anyone able to detect them.
It is called the Search action and is described on page 193 of the PHB:
When you take the Search action, you devote your attention to finding something.
It will pit a Wisdom(Perception) roll from the searcher against the previously rolled Dexterity(Stealth) check from the hiding creature.
This is a general rule. If the DM believes that something should happen a certain way, they are allowed to rule against the book. This, unfortunately, must be done on a case-by-case basis. Pay attention to be consistent, though.
It is reasonable that walking into (the space of) an invisible creature might reveal them. If they wish to let the other creature pass, calling for a Dexterity(Stealth) or possibly Dexterity(Acrobatics) check seems adequate.
