someone's back is turned
(one's) back is turned
One is not looking; 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.
someone's back is turnedsomeone's attention is elsewhere.
1989 Orson Scott Card Prentice Alvin That prentice of yours look strong enough to dig it hisself, if he doesn't lazy off and sleep when your back is turned.