Interested in Language
"Is the militia really going to be called out? The Home Guard, too? I hadn't heard. How do you know?"
I.Why is " hadn't", not "haven't"?
II. Is it correct with "haven't" here?
"Haven't" would shift the meaning to: "I hadn't heard this up to now -- and I still haven't."
"I hadn't seen her until now" means that now you have finally seen her, but "I haven't seen her" means that you STILL haven't seen her.
In the example you gave, if the person said, "The Home Guard too? I haven't heard. How do you know?" it would mean that they STILL haven't heard it -- presumably meaning "from an official source" or "from my superior officer" or "from the media" -- they haven't heard it in some way that is better than the gossip the person is sharing (since apparently his telling you does not constitute your having heard.)
"Oh, really? I haven't heard that" is an expression of disbelief or suspended judgment. It implies that the person's words are not sufficient for you to believe the statement -- that, in effect, you STILL haven't heard it in any way that counts.
"Oh, really? I hadn't heard that" implies that now you feel adequately informed. You hadn't heard it up until now, but you agree that you have heard it as of now.
But really, the difference is slight, and I think that in ordinary speech, a person might use either one to mean, "I'm surprised but convinced."