空穴来风未必无因

lit. without wind there cannot be waves (idiom); there must be a reason; no smoke without fire

lit. wind from an empty cave (idiom); fig. unfounded (story)

lit. flies do not attack an intact egg (idiom); fig. no smoke without a fire

lit. large waves in windless conditions (idiom); fig. trouble arising unexpectedly

lit. grass stirring in the wind (idiom); fig. the slightest whiff of trouble