Condensation occurs when a humid atmosphere is subjected to a heat/cool cycle. If you take an empty gas can and stick it outside a few days, there will be some condensate in it. (That's also why OTR truckers have to drain the air tanks on the brake system occasionally.) Sounds like you had some condensation on the inside of the valve cover.
If one side of the car was closer to the house (or another potential heat source) that side would have less temperature fluctuation and less condensation.
