Most likely condensation - you have a locking fuelcap right? I doubt rain water got in there and if it did it would trickle down into the tank.
Always a good idea to keep as full of a tank as possible in the colder months.
My cap too has some condensation on it when I go to fill up.
1967 Series IIa 109 5 Door - Work in progress
'98 Toyota Land Cruiser 100 Series