A) Space telescopes have very very narrow fields of view. Unless it was tasked on this small spot, it would never see it. This motion would have been first spotted probably with a large FOV imaging device based on Earth. It is moving very very fast. Imagine looking through a paper towel tube, trying to spot an aphid fly by.
B)Again, it is moving very fast. Watch the animation, relative to the speed of the planets. We have no way of moving that fast. Our fastest probes take years sling shoting off of other planets to achieve a velocity a fraction of how fast this is moving. So,we are a long way off.
C) Our solar system is not very dense, yet compared to the space between solar systems it is full of activity. Empty space is incredibly vast. The fact we haven't detected something in the 100 or so years we would have spotted it doesn't surprise me.
If you haven't watched Powers of Ten, I suggest you do, to see just how the scales we are dealing with.
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