For all you, I, they, etc. know
Definition 1 of 1
Pro Tip 1/3Uncertainty Expression
This phrase is used to express that something is unknown or uncertain. 
She might be at home, for all I know.
Pro Tip 2/3Possibilities Open
It suggests that many other possibilities exist because the actual situation is not verified. 
For all you know, he's telling the truth.
Pro Tip 3/3Indifference Implied
Sometimes it implies a lack of interest or concern about the truth or details of the situation. 
For all they know, the meeting could have been canceled.