Even in seemingly private settings, one must be cautious about what they say, as there might always be someone listening or overhearing. It implies that secrets are hard to keep and advises prudence in speech. (Similar to 'Walls have ears.')
他们在房间里低声商量事情，却忘了隔舍须有耳，窗外岂无人，结果计划很快就传开了。
They whispered about their plans in the room, but forgot that the next room might have ears and there could be someone outside the window, and as a result, their plan quickly spread.
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