Islam and "honest atheists"

I have recently reread the quran, and one thing strikes me as odd: withinin the quranic taxonomy, people fall into one of four categories: believers (mu'minun), disbelievers who pretend to be believers (munafiqun), disbelievers who deny god even though they know in their heart that islam is true (kafirun), and those who haven't heard about islam -but no "honest atheists", who work on these issues (existence of god, truth of islam, etc.) but cannot come to the "right conclusions", against the best of their efforts.

What does Islamic literature have to say about such people, and what are the historical reasons why there is no mention of them in the quran?