Not a Neroscientist but here's my guess based on what I know:
You thought about it. Your brain had moved from the conscious section of your brain to a reflex action. The instant you thought about it your conscious mind tried to find it and came up empty. It's not that you don't know it, it's just your brain moved it off "center" (This is such a lay term I don't want to hear a word).
It's sometimes called unconsciously confident in business, the final step of training. The previous step is consciously competent.