I'm having trouble understanding the mechanics of invisibility, so I'd like to propose a scenario, as an example, which I don't know exactly how to handle as a GM.
Let's say my group is about to fight an Invisible stalker. The creature surprises them and a combat begins. At this point, at the very least the character who got attacked knows the location of the creature, right? It's a melee attack after all.
But what happens if the stalker walks around every round? Do they have to make a Perception check to find out on which square of the board the monster is EVERY round?
I guess what I'm trying to understand is exactly when do the characters have to guess the square where they think the invisible creature is, and when do they just know it.
The invisible condition states:
Invisible is not the same as hidden as pointed out in the answer to this question.
Unless a creature is hidden, other creatures know the location of an invisible opponent without needing to make an ability check because "the creature’s location can be detected by any noise it makes or any tracks it leaves".