My guess is that there is two across the top and not a gridwork of them as are on the side.
If you search for vodka cornstarch, you'll see a method of making a paste that will reveal where any lines are. Presumably you could also use a highly sensitive laser thermometer to do this.
Remember that the refrigerator wall is essentially Styrofoam sandwiched between a layer of thin steel and thin plastic. If you like to roll the dice, you can carefully drill the thin outer skin of the fridge wall (either side) , just enough to make a hole. Then you can root around removing any styro to see if there is a line nearby.