Probably because our spy balloons have been watching it since it lifted off

they didn't drop it from a commercial flight?

Or how they know it's a surveillance balloon as opposed to some kind of weather balloon. I guess you could say a weather balloon is still conducting a kind of surveillance.

They mentioned it was already common Chinese practice, it just that this one loitered longer and for days compared to other Chinese balloons.

Maybe it has a big China flag on it?