Imagine you are an Earth-level (0.7 on Kardashev's scale) civilisation on the path of this monster.
In fact, let's just imagine it were to happen in the Milky Way. What happens to us then?
In fact, let's just imagine it were to happen in the Milky Way. What happens to us then?
So if something similar entered our galactic neighbourhood, we would notice changes in the proper motion of stars closer to it, discrepancies between the planets' predicted and actual positions, and (possibly before anything else) predictable errors in GPS timing (since this is probably the most commonly-encountered system depending on accurate knowledge of speed, time and gravity).