Sounds like condensation from large temp fluctuations. If the night temps are prett cold, the oil is cold at daylight, and since oil is pretty thick and blocked from the sun, the outside ambient temp starts raising the oil pan temperature all while the oil is cold. This creates condensation on the oil pan.
Moisture under the cap can be from same thing, along with short duration trips like to the store and back. You can have a perfectly dry internal engine that drips water off the filler cap like yours did when temps change. It's called Dewpoint and creates moisture.