From Longman Dictionary of Contemporary Englishwhen/while somebody’s back is turnedwhen/while somebody’s back is turnedif something happens when your back is turned, it happens when you are not able to see or know what someone is doing What are your kids up to when your back is turned? → back
Examples from the Corpus
when/while somebody’s back is turned• Once, despite the age requirement, my sister charms her way on to this ride while my back is turned. 