Where do unknown knowns show up? Remember the first time you went to a foreign country? When you're in a foreign country you often know the locals know something that you don't know that they don't know they know. Often the unknown knowns relate to non-verbal things or subtleties in language.
Even in England (we recently visited for the first time), where people speak our same language, the use of particular words and the way people say things is quite different. The Americans who have been there for a while know there were things that took a while to get used to when they first arrrived in the UK but often they don't even know what specifically to tell us as first time visitors. They picked up unknown knowledge and don't know how to articulate it but they do know how to use it. There are also known knowns that people might have and relate to you like "oh, a bap is a roll" when the girl at the counter asks if I want a bap or a toastie (what's with the constant use of the diminutive over there, anyhow?). But along with the knowledge they're aware of, there's a whole set of knowledge (in this case cultural) they're unaware of that they use every day. Although some knowledge might be passed explicitly (known knowledge), unknown knowledge is usually passed implicitly and a required ingredient for this passing of knowledge through experience is proximity. How's this for a formula:
proximity + time + a proper human = unknown knowledgeNote: I would say "a properly functioning human brain" but what's a brain without a human attached?
So... if you pick up a book about Africa you might get a lot of known knowledge but if you want unknown knowledge about Africa you probably need to go there.
PS1 - I usually like to write about a slightly different class of topics -- those things you think you know but you don't know (falsely perceived known knowns) but I know you're all tired of hearing that sort of discussion from me and we all know how it goes.
PS2 - yes I'm having too much fun with language at the expense of clarity
PS3 - don't try to track down unknown knowns unless you intend to make the set extinct
PS4 - are there other, better arguments to be made about why we expect our "experts" on Africa or the Middle East to have actually spent some time in those places? But even there I guess it isn't so often a problem with the expert as much as it is with the decision maker (dilbert).
PS5 - Nods to Donald Rumsfeld here btw ;)