Naked eye objects are easy to spot if you know the sky. I know this is a hard concept for you because you don't know the sky like I do, but it's quite easy for someone like me to tell which ones are planets and I would definitely notice if there were a new planet in the sky. Believe it or not, a brown dwarf would reflect as much light as one of our solar system's gas giants.

I notice you ignored the fact that such an object would also greatly perturb the positions of the planets...