when (someone's) back is turned

when someone is not watching what is happening.

When my mother's back was turned, my grandmother would give the dog candy.

• a load of crap
• a load off mind
• a load off your mind
• a load of something
• a lone voice in the wilderness
• a lone wolf
• a long face
• along for the ride
• a long haul
• a long shot
• along the line
• along the lines of
• along the lines of something
• along the same lines
• along the way
• along those lines
• along with
• along with something
• a loose cannon
• a lost cause
• a lounge lizard
• a love child
• a love-in
• a love nest
• a low ebb