Does Chinese have a phrase equivalent to "For all I know"?
I can only give some possible translations based on your explanation of "For all I know":


  For all I know, he might have gone abroad.
  
  他说不准出国了呢。 
  
  For all I know, she doesn't even work there anymore.
  
  她没准已经不在哪工作了呢。
  
  For all I know, the test hasn't even been written yet.
  
  测试题说不准还没出呢。


We would use "说不准" "没准" and things like that to express the uncertainty. I can't think of one phrase that would have the same meaning of "For all I know" as your explained.