In "Death by Landscape," Atwood starts out by describing the paintings in Lois' apartment, noting that they were not peaceful to Lois. Rather, they filled her with "a wordless unease" as if there is something or someone looking back out. The reader is left with the expectation that perhaps the reason for this foreboding will be answered. In the first five paragraphs, all we are given is a lonely widow surrounded by a plethora of landscape paintings, all giving her a wordless, uneasy stare in return. This opening establishes a potential mystery to be answered. 

The rest of the story does just this, recounting Lucy's disappearance. The mystery of the haunting landscape paintings is answered. I interpret the last line with a bit of optimism that Lois at least partially concludes that these landscapes which once haunted Lois now seem more like a windows which stretch beyond the dimensions of space and time. She notes that they are in fact, not landscapes but, foregrounds which retract endlessly away, paralleling the idea that Lucy is "entirely alive" just in spirit or memory.

Lois' actual experience with Lucy when they were kids recedes as time goes on; as does the foreground into the picture. As Lois gets older and her childhood recedes further into the past, Lucy recedes further into the foreground of the pictures; endlessly moving away but always there as if she's hiding always just out of sight. It's also endearing having the image of Lucy hiding under the canoe in the picture as if she's playing hide and seek, another nod to the notion of secrecy and nostalgia of childhood. 

How does "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.