Post History

I don't think that anyone can really answer that without first answering how they *expected* to know in the first place.  Did they plan to monitor the outside world through some technological means?  Did they plan to have someone jump out into a potential war-zone with who-knows-what new environmental hazards every few years and get their news from the first one to return safely?  Did they expect a phone call?

That sounds flippant, but consider what our knowledge of various wars would look like if only communications got cut.  For a lot of us, it'd probably take an active effort to find out what's happening more than a short drive away, and we (presumably) *don't* live several generations down in a forgotten bunker.

The short answer to the question, I think, is just that whatever system that they had in place to find out about the end of the war failed, because most conventional systems will inevitably fail for all sorts of reasons.  The surveillance system didn't survive the war or a fuse burned out.  The people who went topside to check got arrested for public indecency, got eaten by mutant kittens, or ran off.  There's nothing outside to connect to for news.

And it might also matter what kind of war this was.  It's a lot easier to notice the end of constant carpet-bombing than biological warfare, for example.  In the former case, you might need a reason for continued shaking that they didn't account for.