A brown dwarf isn't quite a star, but it's close enough for your LOL to fit. You'd certainly detect one coming from a long way off. By now, it would be visible from the surface of Earth at night, maybe even during the day.
So we're left with four options:
i) Nibiru is fitted with one hell of a cloaking device that also makes it massless or somehow blocks its gravitational field so it doesn't affect anything else. Nibiru is a Stealth Mega Death Star! Man the X-wings!
ii) Nibiru is nowhere near Earth at the moment and will teleport in nearer the date.
iii) Nibiru is nowhere near Earth at the moment and is travelling extremely quickly. Far more than quickly enough to break orbit and go flying off into deep space, but it remains in orbit somehow, in defiance of every theoretical and observed aspect of planetary motion. It also has one hell of a containment field to keep it together.
iv) Nibiru doesn't exist.
My money's on (iv)