go behind (one's) back
To do something in one'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
If someone goes behind your back, they do something secretly or without your permission. Leonard, you haven't been completely open with me. You think I wouldn't know when you go behind my back? They go behind our backs, they withhold information, they talk down to us like idiots. Compare with do something behind someone's back.