Early in the story, one flew in front of the house does the narrator observe?

Are you sure you don't mean flaw? If so.....

Perhaps the eye of a scrutinising observer might have discovered a barely perceptible fissure, which, extending from the roof of the building in front, made its way down the wall in a zigzag direction, until it became lost in the sullen waters of the tarn.

A barely perceptible fissure