In "Half-Life," how does David's preoccupation with his grandmother's words as he walks home from the swamp foreshadow the fact that she has died?

As David walks home from his night in the swamp, he thinks he can hear his grandmother's voice telling him to be good. He thinks that her voice is "everywhere but nowhere" (195). Later, he will find out that his grandmother has died, and so in a sense she really is "nowhere," but since he carries her words inside him, she is, in another sense, "everywhere."