Walls have ears, and there are always people nearby; meaning one should be cautious about what they say, even in seemingly private settings, as secrets can easily be overheard.
你们谈论这件事情，最好小声一点，记住隔墙须有耳，窗外岂无人，以免不必要的麻烦。
You'd better discuss this matter in a low voice; remember that walls have ears and there are always people nearby, to avoid any unnecessary trouble.
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