Unless it's from someone even more powerful then you.... who knows, if there really is tech to both move AND hide a brown dwarf, then there is likely tech to both move and hide even larger stars. Possibly even tech to hide/move systems. Another reason to hide: if I was an alien species I would observe a new species I would observe them in their natural enviorment, without revealing myself. What if this supposedly primitive new species turns out to be a powerful species hiding as weaklings, and of course, what if you decide you don't want to talk to them? Best they never knew you existed. That's assuming A) Niburi is real, B) Aliens played a part in moving it, and C) they would want to hide. I do not beleive in any of these things.
rough physical size of a dwarf star????
bigger then earth but half the size of the sun maybe even a lil more.
A brown dwarf is a large ball of gas that never had enough mass to create nuclear fusion...a failed star...no fusion, no star. They're about the size of Jupiter up to 10x the size of Jupiter...still tiny when compared to the sun, yet large enough that I can assure you that nobody's driving anything that size anywhere. Large enough that there would be no way for it to sneak through our solar system unseen, while not causing a perturbation in the orbit of a single planet. Mercury orbits the sun every 88 days... but somehow it whizzes by Nibiru every three months unscathed.
Additionally, something as small as a brown dwarf (less than 1% the mass of the sun) could not defy the laws of physics that all other bodies adhere to. If it ain't orbiting, it's crashing into the sun, assuming that the solar wind doesn't blow the gas from the brown giant back into the outer solar system. Ever notice how all the gas planets in our solar system exist outside the orbit of Mars?