Very good point about "I don't know" meaning exactly that! I have tried to use "I don't know" only when I don't, and for other times when I might use that phrase as to mean something else, like I'm a bit confused, or I'm reluctant to state 'this', then I try to use something else, like a draw out "Welllll" instead. Because yes, "I don't know" certainly can mean just that!