Having read plenty of Dr. Oliver Sacks (e.g: Man Who Mistook Wife for Hat), there has almost-certainly been a savant ("idiot," or not) with this actual ability – and nobody knew how it worked (it just did), similarly to the documents Sacks-patient that immediately knew how many toothpicks were on any table (in the hundreds-to-thousands)... but could otherwise not talk.

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To explain this with a DCC metaphor:

NEW ACHIEVEMENT: All prime numbers now glow bright orange, including entire mobs (if they contain prime number of living members). This includes those cute little toes of yours, Carl. Don't question this, it just is.

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My favorite Sacks patients were the non-verbal aphasics laughing at Nixon's public TV appearances (because they were experts at telling if people were lying [by laughing at non-verbal cues]), otherwise disconnected from society.