Scientists rarely rule anything out completely. That said, I expect it to be **highly unlikely.**
Recall that [Pluto was hypothesized in the late 1800s](https://en.wikipedia.org/wiki/Pluto#Discovery) as the source of perturbations observed in the orbits of Uranus and Neptune.
Our measurements of the orbits of even the outer planets are almost certainly significantly better (and we certainly have data covering a longer period) today than they were 120-130 years ago, yet the measurements of the time were sufficiently precise to deduce the presence of a quite small celestial body (less than 1/5 the mass of Earth's moon) quite far out.
There is also the [Pioneer anomaly](https://en.wikipedia.org/wiki/Pioneer_anomaly), where we observed, and subsequently were able to explain, acceleration caused by *heat loss* in the Pioneer 10 and Pioneer 11 probes.
If there were additional, as yet unknown, large masses in our solar system, those masses would cause unexplained perturbations in the orbits of the other planets. It would probably take a fair bit of investigation to determine that the cause actually is an additional planet, but any reproducible result pointing in the direction of such perturbances would likely be of great interest. It might not quite be at the level of a Nobel Prize, but it would certainly earn the people involved great accolades within the scientific community.
Also, orbital mechanics is sufficiently well understood by now that we should be able to relatively easily determine, based on perturbations observed over a reasonable period of time, both the location, mass and orbital parameters of the body causing the perturbations. With our current selection of telescopes, both ground-based and space-based, operating both inside and outside the visual part of the EM spectrum, it should be relatively simple to then conclude that there is *something* in that location even if that body neither emits nor reflects any radiation (which it would), and based on stellar occlusions likely relatively easy to estimate its size. Given size and mass it's trivial to derive average density, which with reasonable assumptions can tell us a fair deal about the composition.