Walls have ears
Definition 1 of 1
Pro Tip 1/3Privacy Caution
Use this phrase when suggesting that confidential information might be overheard. 
Discussing our strategies here is risky – walls have ears.
Pro Tip 2/3Conversational Warning
This phrase is a discrete way to advise someone to speak less freely. 
I would keep my opinions to myself if I were you; walls have ears.
Pro Tip 3/3Hostile Settings
Especially relevant in situations where mistrust or surveillance is a concern. 
In this competitive industry, don't forget that walls have ears.