A proverb warning that someone might be listening (literally 'walls have ears'); used to warn against speaking carelessly or revealing secrets.
会議の内容は外に漏らさないで。壁に耳ありだからね。
Don't let the meeting details leak outside. Walls have ears, you know.
喫茶店で会社の悪口を言っていたら、壁に耳ありと後で後悔した。
I complained about the company in the café and later regretted it—walls have ears.
秘密の話をするなら、ちゃんと個室で。壁に耳ありだからね。
If you're going to talk about something secret, do it in a private room. Walls have ears.