Given that it was only within the last 15 years when telescopes with “larger” mirror sizes than the Hale telescope on Mount Palomar started to find planets beyond Pluto at least Pluto’s size, we’re still only at the beginning to finding a potential brown dwarf companion to our Sun. With new, more advanced satellites going up within the next few years, we may finally get the technology to find it.
The problem with finding it used to be that the various hypothesized locations were surveyed and nothing was found. IRAS covered quite a bit of the sky and didn’t find any telltale infrared. If the companion is in a polar orbit, it probably would be missed, an idea which came from the discoverer of Pluto, who continued to survey the skies for years after his discovery. Perhaps the construction of those big scopes in the Andes (the southern hemisphere having much less landmass and population, some of the sky had been neglected) and these orbital observatories will find something. Hey, if everything was known, there’d be no reason to build stuff like that.