Observation differs from seeing: most people register that something is present without extracting what its details imply. A worn hat, a scratched watch, or mud on a boot carries a reconstructable history of its owner's habits, finances, and recent movements if each feature is read as an effect with a traceable cause.
Once the impossible explanations are eliminated, whatever remains, however improbable, must be the truth. This inverts intuition: a bizarre-seeming solution should be preferred over a mundane one if the mundane ones are each ruled out by the facts.
In 'A Scandal in Bohemia,' Irene Adler defeats Holmes by recognizing his disguised reconnaissance, then flees with the incriminating photograph after leaving a letter revealing she saw through him. Holmes, who habitually dismisses women's reasoning, afterward refers to her as 'the woman' and keeps her photo, treating his single defeat as more memorable than his victories.
The ordinary and respectable surface of domestic life frequently conceals the gravest crimes, while the lonely countryside, assumed safe, may hide more undetected cruelty than the crowded city because isolation removes witnesses. Appearances of normalcy should raise suspicion rather than reassure.