IO540, could your wings, unlike the rest of the aircraft, be a lot colder than the minimum night time air temperature? Perhaps the ground, (which cools the air), and the wings undergo radiation cooling at different rates?
It is very noticeable that car roof tops frost over more than the sides of cars, unless the car is tilted (eg on a cambered road) when the part pointing at the sky is the most frosty. You can see similar effects when there is a tall building on one side of the car.
I think it is simply because the supercooled water droplets are coming down more or less vertically, unless modified by some local airflow, and then it freezes up to create frost.
That's why one doesn't get frost underneath the wings, IMHO.
The wing tanks will definitely be colder. If I turn up to fly at 9am and it is -2C, I bet the fuel will be at say -5C from the middle of the night.
it was discovered that you'd used a non-approved de-icing method
I thought that an American Express card was approved for this purpose; it seems to be widely used by instructors
Personally, I would use a Santander card - this is the most arrogant institution around today.