It's more like what chance would they have of missing it.
If it existed and it had an orbit of ~3500 years and it was less than 2 years away, it would be relatively close by now and it would be very obvious indeed. Even if it was a dull, rocky planet, it would be absolutely impossible to miss.
Also, if it did exist and it was here in 1500BC, it obviously didn't do any serious damage then so why would it do so next time?
I've seen people seriously claiming that Nibiru is a brown dwarf. Really. if that was the case and it was less than 10 years away, it would be the second brightest object visible from Earth (after Sol). It would be brighter than the full moon. 2 years out, it would be far brighter than the full moon. Everyone who could see would be seeing it even at midday in bright sunlight.