Maybe... it hides behind both the Sun and Jupiter. They're both big, so there's even more space to hide in that way.
Planets aren't stationary.
And if a planet can't be found by NASA, then how the heck did some conspiracy theorist nutjobs find out about it?
But lets say that there is such a planet. You still haven't explained how it's supposed to go from "behind" Jupiter to Earth in just three days.

The whole thing just doesn't make any sense at all. I could go on and on about why it just... Oh my god dude... What the heck?! The physics of it just makes no sense!
We're just mere musician here. Not scientists. Unless...
False authority fallacy?
For some time I actually wanted to be a physicist when I grew up.
ANYWAYS...
I know what you're talking about. Apparently there's this planet with a highly eliptical orbit of 3600 years called "Niburu", and it hosts an alien race which actually aren't aliens because the engineered us, and they're going to fly by and turn us into slaves... Combined with Nimburu colliding with us... Combine with the entire Earth flipping over 180 degrees... Combined with all the planets in the solar system aligning with the sun and the dark rift in the center of the galaxy destroying the Earth... Combined with solar flares burning the Earth to a crisp.