I don't know if this has been mentioned yet or not (it wasn't on the list though), but couldn't you say that Harry's skill at Listening is an unsolved mystery?

So far, we know that:

Harry aquirred the skill from time and practice
The Gatekeeper has it as well:

"I understand you know how to Listen, too." (SK, 42)

But other than the times when Harry and the Gatekeeper use the skill, Listening is hardly (if at all) mentioned in the series. Do other wizards have the potential to learn how to Listen, and do they even know about the skill?