This question's solution says that as long as any part of the creature or clothing its wearing makes a noise it can be detected. A Will-O'-Wisp is ephemeral, has Incorporeal Movement,and flies. With all this going for it a Will-O'-Wisp should be undetectable while invisible without needing to hide unless, it has a smell/scent, Incorporeal Movement leaves a detectable trace in the air,it or its movement makes noise.
Players Handbook PG 177, Hiding says "An invisible creature can't be seen, so it can always try to hide." which indicates to me that unless an ability explicitly says you become hidden that you're always detectable in some way outside of your control.
I feel like I've come to the correct conclusion but would like confirmation or an explanation if i'm wrong.