Tā yǐwéi méi rén zhīdào, méi xiǎngdào dōngchuāngshìfā.
He thought no one knew, but then the whole thing came to light.
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From the Yuan play 《东窗事犯》 and later Ming geography 《西湖游览志馀》: Song-dynasty traitor Qin Hui plotted the murder of general Yue Fei under the east window of his home with his wife. After dying, Qin Hui supposedly sent a message from the underworld: 东窗事发矣 - 'the east window affair has come out.' The idiom has meant 'the plot is exposed' ever since.
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Always about wrongdoing - crimes, corruption, affairs, fraud. Strongly negative (from the criminal's perspective). Natural in news reports and narratives, common in crime and political contexts.
