Air frost also described as hoar frost can only form (according to the books) on the ground ie in static conditions. The static air on and over a very cold aircraft surface is cooled below its dewpoint and condenses out to form dew and if its below 0c the dew becomes frost (ie ice).
There is generally no hoar frost on a wing in the air at night or day because the air is not static, its rushing past at 100kts etc so has no opportunity to adhere to the aircraft.
I am not aware of any aircraft ever icing up in flight in clear air with hoar frost.
Remember at night many types of precipitaion/clouds cannot be seen even when you're in them, virga (rain not striking the ground) is the best example. If you ice up in the air at night it will be because you are in some form of moisture however it would be visible if it were day time.
Finally, I did say generally as Sycamore is correct that a type of hoar frost can form under some airline jet wings due to fuel soak but the temperatures involved are in the minus 60s at very high altitudes. Just to correct Sycamore it does not have to be below 0 on the ground. MY aicraft will show ice under the wing at 20c as its the fuel temp thats critical here not the outside air temp.