Nor is there a rule that states "you are hidden unless you are detected" in 5e. You are never hidden in combat unless you successfully take the Hide action in 5e.
If the invisible monk is in the area of a combat, unless they hide successfully or get behind total cover or move out of range (perhaps to the other side of the planet?) on their turn, they can be attacked per the invisibility rules. Hence they are detected (or "heard" or "locatable") in some way, making it possible to target them with said attack. I don't think anyone is arguing the mechanics of that. To me,
@Flamestrike and
@Ovinomancer are just arguing about the fluff of how one might describe those mechanics in the scene.