Dude, chillax. I didn't call you out.
They lay down a beacon, it could be years before anyone finds them. They'd have to be within range for her to make that statement, whether it be in range of a colony or Earth. Since most all the colonies are gone, Earth's the only viable choice I see.
The Forerunners are dead, I'm pretty sure. We're their ancestors. I believe it was Guilty Spark and the Gravemind making references. "You ARE Forerunner." And "A Father's sins, passed to his son."
Why would they leave planetside to investigate? On their home turf they know what all they have and how to fight them off. And that world would probably have some automated defensive structure that responds in action. "Dawn 1/2" doesn't seem very hostile, so why bother? Sit back and scan it, then proceed with caution to destroy it or grab it.
Who knows how the Forerunners thought?
When cities burn and armies turn and flee in disarray,
Cowards will cry "tis best to fly and fight another day."
But warriors know it, in their marrow when they die and fall:
It is better to have fought and lost then ne'er have fought at all.