Retrieving information: how can we know so fast that we don't know something? If someone asks us something that we don't know, how can we know so fast that that's the case? Shouldn't we spend a considerable amount of time scanning our memories until we realize the information is unknown?
I am not an expert but I think this is because the brain would find existing memories instead of looking for memories that might not be there.

For example:

Say you have a good knowledge in C# but don't have that good of a knowledge in C++, but you know the extent of your knowledge.

You'd immediately know you cannot answer a question when asked about a concept in C++ you have not heard of / are not up to yet.

Another example:

When asked where you last saw someone, you can remember quickly or you can't. 

For example, you have a friend that went missing unexpectedly but when you last saw her the confrontation was so casual you can't even remember it.. except that you can.. just slightly.

If you see your friend and give them a quick wave, but are in an area you aren't familiar with (Say a shopping district you've never went to), you'd have a difficult time recalling the PLACE where you found them. This is especially true if you're just acquaintances who just say "Hi" and "Bye!"

But if you are in a grey area, say you've been to a certain place before and saw your friend/acquaintance there, you may not remember or you will remember, that is just subjective. You'd be likely to remember quicker if it was a large event between you two. (Like a wedding!)

All in all, usually when asked that type of question images will flash by while you're saying "Hmm.. umm.. well I think I last saw (him/her) at this place." or you can be unsure and blank out.

Sorry for my inexperience, I am just giving an example to explain what I cannot really formally write.