Why is Grace Poole always hiding on the third floor?

I think Grace is always on the third floor because she has a secret. Some sort of secret is referenced in chapter 17, when the servants are talking about how she is paid so highly. It also made me think Mr. Rochester has something to do with this secret when she was not punished in any way for almost burning him alive, and completely scorching his room. I believe she is being paid to stay out of everyone's way, on the third floor, but yet still be under Rochester's control. Maybe from some affair in the past, I am not really sure why else she would stay up there when she is getting paid so much.