Humidity on oil cap

You never got the oil hot enough for long enough to boil the water/moister out of the system. Cover the oil cooler partially and let the oil heat up to normal operating temperatures if you are just doing ground runs to get rid of the moisture.

I am having the exact same experience. The dipstick got quite corroded, and there is significant condensation on the cap. And I don't even live in a humid environment. I can understand the condensation, but the corroded dipstick seems completely out of line.

https://www.aircraftspruce.com/catalog/eppages/xcomthermostat.php

Short-term ground running is not good for your engine -- such short-term / low temperature operation only serves to reintroduce and activate moisture within the engine's oil.  This entrapped moisture will corrode / rust your engine's internals. Virtually all aircraft engine manufacturers suggest that a period of 30 - 45 minutes of in-flight operation at normal engine operating temperatures is required to fully drive the moisture out of the oil.  A minimum oil temperature of @ 180 degrees F is typically desired -- this temperature means that at some point within the engine the oil is experiencing at least 212 degrees F (boiling point of water).  If you find a light-brown frothy mixture inside your oil cap that is an indication of excessive moisture in your engine's oil.

Ken Stated:
" ...  And I don't even live in a humid environment... "

You may not live in a humid enviroment, but your engine most certainly does.
As the Hydrocarbons of the fuel are burned, they convert mostly into CO2 and Water.
It is not unusual to see water dripping out of automotive tail pipes during cooler weather.
Each gallon of gasoline produces almost a half-gallon of water in the exhaust vapors.

It is the exhaust gas that leaks past the piston rings that drives the Oils return to the tank.
Some amount of water in the Oil is completely normal in all internal combustion engines.
You just need to get the oil up to at least 200°F (95°C) on a regular basis to keep the water content within limits.
And another reason that letting an egine remain unused for extended periods is not recommended.
If you see something dripping out of your Oil Vent Hose, do not automatical assume it is Oil.
It might just be water.