What was the speaker anticipating and why in the above lines "a door had lately been sealed and shut in that staircase"

This event occurs in Chapter XV (p.13). In this scene Jane is the narrator/speaker, and she has heard a demoniac laugh from behind the wall. She then hears footsteps going up to the third story staircase that has been sealed. Jane anticipates that it is Grace Poole, and she asks herself, "is she possessed with a devil?" (13). Jane does not know that the laughter comes from Rochester's first wife as that has been the secret kept hidden from her. Rochester's wife has been kept hidden in the house.
It is just after this passage that Jane smells smoke and follows it to Rochester's bedchamber. The curtains around his bed are on fire, and Jane douses the flame with water to put it out. By the end of this scene, she is still unaware that Rochester has a wife hidden in the house who started the fire.