go behind back
go behind (someone's) back
To do something in someone's absence. The phrase typically suggests wrongdoing or deceit. I can't believe you went behind my back and gossiped about me! Tom will be upset that we've gone behind his back and made the decision.
go behind someone's back
1. Lit. to move behind someone; to locate oneself at someone's back. The mugger went behind my back and put a gun to my spine. Bob went behind my back and pushed me through the opening.
2. Fig. to do something that is kept a secret from someone affected by it. I hate to go behind her back, but she makes so much trouble about things like this. Please don't try to go behind my back again!