Well im a nihilist (or at least i like to call myself one) so i dont really 'believe' in anything as such, although ghosts are something i come very close to believing in - mountains of evidence in their favour; photographic, video-tape, film, eye-witness accounts, group eye witness accounts.
The evidence is there really, the central problem is there's no theoretical scientifc model to support them. And people will always confuse a lack of a theoretical basis with a lack of evidence.
So it's not so much that we can't 'detect' ghosts, thats never been the problem, its that we cant account for them.
You can see the exact same 'data-blindness' if you look in scientific history at things like sprites or fireballs. These were all things that people had simply 'hallucinated' untill we had a proper framework with which to understand them.