when fading to the background i envision a bubble forming around certain topics allowing one to use and reference them unconsciously at a macro level, but leaving the micro information hidden.
an example would be when i worked in games and did a ton of 3d math to animate and move objects around the world. at the time i was a gameplay programmer these ideas where at the tip of my fingers. i didn't have to think twice about matix multiplications, dot products, vector projections.
currently i work on ios/android development. i don't use 3d math much when deving any more. if you were to quiz me, i may not be able to implement or use those ideas very easily. a few weeks working with the same topics would bring them back to the forefront.
i learned all the sorting algorithms in college. i could probably implement a couple, but most of them i don't recall immediately or at all since i rarely care about the efficiency of the data i sort these days. i know of their existence, but to a degree that is all. if i were in a position where sorting was important, i would make sure to educate myself to appropriately.
i google almost everything. there are rarely days when i don't google something for work. learning is infinitely fun. i think being able to research a problem and find a solution is as valuable if not more as already knowing the solution.