You'd be surprised how much stuff there is just in our neighborhood that we're unable to catch, yet. We mostly track dangerous stuff, the big enough - fast enough to destroy us, and we now have probably in the 6-7 figures objects tracked in the inner solar system. That's it. Elusive 9th planets and other major 'quests' are shots in the dark so far, mostly — read accounts on why they target some region of the sky, how narrow our observational capabilities are as we can only point to a very specific direction at a time with each telescope, etc.

So, some alien unmanned probe the likes of Voyager in large orbit around the Sun? Yeah, no chance in hell we'd detect that by random chance, we'd have to seriously upgrade our space infrastructure to even pretend 'monitoring' our close vicinity, let alone post-Jovian space. And we have virtually no knowledge of about half the distance between interstellar medium and the Sun, that's how limited we are to even modeling the closest star systems.

Edit: apparently double-post with aruggirello; I agree with him.