As Dualcouple says, on the A320 the problem occurs on turn-rounds in dry conditions of cold humidity. The mass of structure associated with the main L/G mounting points - sometimes referred to as the Bermuda triangle - retains its chill from the previous sector and conducts it to the adjacent skin on the top surface of the wing, causing frost to form. Back in the last century, at some of our outstations, the station engineer had a portable de-icing kit to remove the frost in the small area.affected.- unless the wings needed de-icing anyway.