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The answers lie in the details that most people look over.
They see, but they don't understand what it means, how to connect the information from their eyes.
The mud on shoes normally kept pristine. Foreign money, tan lines, an irritation from a shot on a person who apparently hadn't been abroad. Second autopsy and internet purchases leading to death by botox. A pinpoint of light that shouldn't exist in a starry sky that never existed.
These were little things, but anyone could see them. They were all obvious for anyone who bothered to look.
Most people see. Sherlock observes.