Gaining a skill after some amount of time (while not actively practicing the skill)

The terms

subconscious learning

or

subliminal learning

are essentially the phrases for what you mean - I don't know if there's a better single-word.

(Simply google for literally 10,000s of both Real Scientific mentions of this, and Crass Commercial mentions, trademarks, etc, eg .. http://www.realsubliminal.com/how-subliminal-learning-works http://effortlessenglishclub.com/subconscious-vs-conscious-learning

Finally: I believe there's a lot of research in to issues like "How many hours per day should concert pianists practice?" where the answer is along the lines "surprisingly the hours when you stop practicing and do something else are where you Actually Learn", etc. So perhaps you could pursue that type of field and find some neat terms being used.

Also Chris as I mention in a comment, it's common that (whether today or historically) some famous musicians have taken the point of view that they "never had to learn", were simply "learning something they already new", or "already knew everything as soon as they were about to be taught it." (This can indeed veer to a sort of mystic re-incarnation vibe.)

An example of this from an extremely popular bestseller of the "new-age" era is the actress and musician Leslie Parrish's explanation of her musicality in her boyfriend Richard Bach's bestseller "Bridge across Forever"...

"But there is one funny thing. Music that's no later than Beethoven, than the early 1800s, it's as if I'm relearning, it's easy, I seem to know it at first sight. Beethoven, Schubert, Mozart-like meeting old friends. But not Chopin, not Liszt . . . that's new music to me."

It could be your feeling leans to this? Rather than "scientific" "subconscious learning, let's say.

So, I'm pointing out there's kind of TWO phenomenon along the lines you ask, Chris,

(A) "scientific" for want of a better word, subliminal learning, which you can find studies about and so on

(B) particularly since you mention music, the phenomenon where certain musicians have reported they "already knew it" - which can be a little "new age", if you will.

So, I don't have exact terms for those two, A/B, but I'm just adding that as a kind of clarifier!

For another more specific suggestion (example, http://en.wikipedia.org/wiki/Sleep-learning ) you could possibly use the term

hypnopedia

in a related way. You could say something like:

"It's so weird, I worked with the desk once, months ago, and never thought about it again. Then today - I'm a master of it with no further effort. It's like hypnopedia or something!"

(BTW, as always with wiki, I don't know if some idiot just made up "hypnopedia" out of fresh air to make that page look good, or, if it is really used widely.)

Hope it helps!

Congratulations, you have grokked it.

grok (transitive, slang) To have or to have acquired an intuitive understanding of; to know (something) without having to think

You could say that you learned it by osmosis:

an ability to learn and understand things gradually without much effort