In D&D 5e, the description of hiding (PHB p177) says an invisible creature needs to stay quiet and otherwise avoid leaving signs of its passing, while the "invisible" condition (p291) says the creature's location can be determined by any noise it makes. This seems straightforward: even a blinded creature can pinpoint the location of anything on the battlefield that isn't tiptoeing/holding its breath.
However, the Ranger's "Feral Senses" ability (p92) grants awareness of the locations of invisible creatures within 30 feet, provided the creatures aren't hiding and that the Ranger can both hear and see. I realize the ability also removes the disadvantage to attack rolls against invisible creatures, but the wording and limitations seem to imply that awareness of the location of an invisible, unhidden creature isn't a given.
I realize the hidden vs invisible question has come up before, but I'm hoping someone can explain what changes for a Ranger when she hits 18th level. Is there a scenario where a creature who can see and hear would not be aware of the location of unhidden, invisible creatures within 30 feet? Or is this just the equivalent of giving a fighter the ability to add half of their proficiency bonus to attacks with simple weapons for which they don't already have proficiency?