wall of silence
1. An unspoken policy of concealment by police regarding the wrongdoing or illicit activity of other officers. A variation on the phrase "blue wall of silence." I respect police, but it's hard to trust them completely when so many stories of the wall of silence come to light.
2. Constant silence, often due to unhappiness or secrecy. Ever since my husband found out that I lied to him, there's been a wall of silence between us. Mom can never know that I broke her vase, OK? There's a wall of silence, starting now.