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Sherlock was stunned speechless when he first saw Irene Adler.
Might've been because of her clothing– or lack thereof.
When he tried to read her, he drew a blank. Sherlock Holmes doesn't draw blanks.
He observes, and draws logical conclusions based off of his observations.
He knew wasn't losing his touch – he read John like a book, deducing the tiniest fragments of his life in a brief glance. He stared back at Miss Adler again. Nothing. She had no disguise. She hid nothing. Miss Adler left nothing to the imagination.
Yet, for all he could see, he could read nothing.