yep the founder of BOS (Col Maxson I belive) worked at the Mariposa Millitary Base (where the vats are) and if you look at his journal using the main computer at the vats (only in FO1) you find (among other things) that he and all other soilders there ceaded from the Union, and they transmitted thier message but never recieved a reply. Also thats very true most of the people in NORAD probably would have survived and tried to contact the nearby millitary bases (sattalites?) such as the Air Force Academy, Edwards Air Force Base, SAD, The Glow, Mariposa, etc. But was the place where BOS setteled after the exodus a founded millitary base in that world? if so theres a very good chance of NORAD contacting them and getting a reply and I agree with you this is probably what happened. And if this is the case a side mission for Denver area would be to go to Norad and make human to human contact b/c as far as we know this is the first time BOS as been near Denver NORAD could also be a in-game resupply point and more game info point, and an all around fun place to visit and explore.