We’ve had the big telescopes all trained on it since we first picked it out of the background. We should have seen it earlier, but we always expected an incoming alien ship to decelerate as it approached the Solar system… it should be coming at us drive flame first, brighter than any star.
This one is still under thrust towards us, flame occluded by the bulk of the ship, which is considerable. It’s not slowing, it’s still accelerating, and the math is clear: it’s going to hit Earth at about 1/2 c. Unless they change course. Here’s hoping they do.