How does Margaret Atwood’s “Death by Landscape” build secrets, silences, and mysteries into the narrative?

This impressive short story by Margaret Atwood contains
one central mystery that is explored and resolved through the flashback of Lois to the
loss of her childhood friend, Lucy, in the Canadian wilderness. The strange and
mysterious disappearance of Lucy in the wilderness, with no signs of her remains ever
being found, is something that impacts Lois profoundly in the rest of her life. Note
what we are told towards the end of the story as we return to the adult
Lois:

She was
tired a lot, as if she was living not one life but two: her own, and another, shadowy
life that hovered around her and would not let itself be realised--the life of what
would have happened if Lucy had not stepped sideways, and disappeared from
time.

This experience has
affected her so much that after Lucy's disappearance she never wanted to go "to any
place with wild lakes and wild trees and the calls of loons." However, in spite of this
deliberate avoidance of the wilderness, within herself she is always listening for some
kind of "echo," presumably referring to the voice of Lucy. It is only at the end of the
story that she reconciles her loss of Lucy and the wilderness that is within her by
finding Lucy in each of the paintings that hang on her
wall:

Everyone
has to be somewhere, and this is where Lucy is. Sheis in Lois's apartment, in the holes
that open inwards on the wall, not like windows but like doors. She is here. She is
entirely alive.

Thus the
mystery of the text is resolved, as by finding Lucy in each of her paintings Lois
acknowledges the importance of the wilderness in her own life and is able to reconcile
herself to both the loss of Lucy and the wilderness element that exists within
her.