when/while (someone's) back is turned

while someone is somewhere else or unable to notice what is happening.

When my mother's back was turned, my grandmother would give me chocolates.

• There's many a slip twixt cup and lip.
• There's many a true word spoken in jest.
• there's method in madness
• there's method in someone's madness
• There's more than one way to skin a cat.
• there's much to be said for
• there's much to be said for doing something
• there's much to be said for something
• There's no accounting for taste!
• there's no law against
• there's no law against doing something
• there's no law against something
• There's no peace for the wicked!
• There's no rest for the wicked!
• There's no smoke without fire.
• There's no such thing as a free lunch.
• there's no telling
• there's nothing to it
• There's nothing to it.
• There's no time like the present.
• There's no time to lose.
• There's no two ways about it.
• There's one born every minute.
• There's one for the books.
• There's safety in numbers.