If you answered no, then congratulations you probably deserve a place on the plot development team for discovery. Go forth and develop many nonsensical plot lines.
Because the Nomad that destroyed Sol was huge, bigger then the planet they were fighting over if I remember right (Haven't seen the intro in a while)
When whatshisface told the governments about the giant alien, they figured that they would be able to detect something that big if it even got anywhere close.
But the thing is, Nomads of today's standards are minuscule compared to that mega-weapon, and not as easy to detect, hence why the Governments were caught off guard.