What this means to any 'lurking' brown star in our system, it would leave a decided gravitational wake, which would perturb the orbits of planets around the whole system in a manner that would pretty much pinpoint the location of a massive body.

Secondly, they are not black bodies, they do radiate and reflect electromagnet energies across a wide spectrum including light. Yes it is possible to have missed a large body with optics even today in the outer solar system, however the mapping of the whole of space with radio telescopes has been done and continually gets updated all the time.

A near brown dwarf would have signaled its presence via radio waves. And yes there are amateur radio astronomers out there.

Yes over long distances their radio power is too low to pick up more than the faintest of signals. However anything within the solar system is close enough to be detected.

We can still pickup the signals of Voyager craft and their transmissions are infinitesimally small compared to what a modest small brown dwarf can produce.