Walls have ears.

Zdi mají uši.

Even the walls have ears, and that lady standing next to you tapping on her Blackberry might just be a reporter. (Google)

Do your walls have ears? It's a scary world out there, with a threat round every corner – and not just for characters in spy movies. (Google)

It is not good to speak of such things ... stone walls have ears. (Googlebooks)

» Communication