I might not be able to help much...
But I can relate to what your daughter expiriences... Well partially! At night I can hear is if someone is walking downstairs, yet there is no one! I tend to check behind my back as if I feel like someone or something is behind me. I feel paranoid in this way.
The best you could do is to take her to a good psychologist I think, it's not fun for her to feel this way, believe me!
A good psychologist shouldn't laugh at his or her "patients" (if I can say so).
What you can do for now is attempt to help her as you see fit, for me it helps to have company of someone I can trust, or an animal to make me feel secure.

Take care and good luck with your issue!