Comment Apollo asteroids most likely (Score 1) 135
If there's a probe of extrasolar origin somewhere in our vicinity, and it's tens or hundreds of millions years old, it would probably not be limited to observing only one planet. So, it would most likely be on an eccentrical orbit, like on an asteroid with its aphelion out by mars and a perhelion by the orbit of venus. I believe there are a couple of those, like 3361 Orpheus.