Condensation. :(
Air carries moisture. Warm air can carry more than cold. On cool nights the temperature of the engine metal dropped below the dew point of the air inside. Tarp doesn't protect against that. :(
However its an important principle to keep in mind: The amount of moisture held in air is dependent on pressure and temperature. That applies to fuel as well as water and so applies to wet intake manifolds when they go from low pressure to high. Just tuck that away for now. :)