I hear a ghost of symbolic evocation here. In the "silence" of the "lighted and empty sala," representing the morally empty material interests, the long-dead voice of skepticism repeats that idealism "left" (i.e., was separated from) its sheltering dream in this room.
As usual in Nostromo, the line is subtly misquoted; Decoud's original
was "'Antonia has forgotten her fan in here."