when your back is turned
when (one's) back is turned
When one is not looking; when one is distracted, absent, or involved in something else. Jimmy grabbed another cookie when his mom's back was turned. It turned out that the vice president of the company had been orchestrating a hostile takeover when the president's back was turned.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
when your back is turned
If something happens when your back is turned, it happens when you are away or involved with something else. The problem is, I never know what she's going to do when my back is turned. Note: You can also say as soon as you turn your back. They stand there, very quiet and polite, but as soon as you turn your back, they go crazy. Compare with turn your back on something.
Collins COBUILD Idioms Dictionary, 3rd ed. © HarperCollins Publishers 2012