Proverb warning that secrets may be overheard; often used as a standalone caution. The fuller form is 壁に耳あり障子に目あり.
See also: 壁に耳あり障子に目あり
Walls have ears, so let's not talk about secrets here.
「壁に耳あり」って言うだろ。気をつけたほうがいいよ。
You know the saying 'walls have ears.' You'd better be careful.
A Japanese proverb equivalent to the English 'walls have ears.' The exact origin is uncertain, but it reflects a common folk warning about the risk of eavesdropping.