A lot a random space stuff flies into the earth’s atmosphere and burns up. So much in fact, that those organizations with the equipment capable of detecting that matter don’t generally feel alarmed when it happens.
I imagine that these beings that brought humanity to heel were detectable by some means before their descension upon our unsuspecting race. Likely ignored just like everything else, why wouldn’t they be? Until they didn’t burn up, until they impacted the ground like rods of brimstone, leveling miles of ground around their impact sites. Those who died in this way, were lucky, quick… first. They didn’t have to see what came after.
Drones and aircraft were sent over these areas of course, they didn’t last long. The feeds would go dead without catching so much as a glimpse of anything strange. Other than the sundered earth. It took almost a day I think, for *them* to reach any inhabited places, they don’t move particularly fast. I think it’s because they don’t feel rushed, this was an attack at their leisure.