If you look at the recent exoplanet photography, you will see, in some cases, planets much farther from their stars than Neptune and big giants.

Also there is the interestelar space, and gravity bound stars... even brown dwarfs can be there, while it couls be a great gravital issue. Who can say what there is there?

1. beyond Pluto would be very difficult to spot even a big gas giant like Jupiter, as at such distance the translation and the movement across sky is very small to be perceived casually.

2. In the outer Pluto orbit the orbital excentricity may be very pronounced. Imagine that such planet could be anywere, not only in the ecliptic (zodiac area).

3. Excentricity also can put is much distant place for hundred of years.

4. Also the brightness would be smaller than 13rd magnitude, ie. beyond most of amateur telescopes.

So I'm not so confident that we had found anything on the outer solar system. While we had much tech advance on Astronomy, we are still very limited to find things if not by accident.