You would be suprised how much work they can do in one night!!! probably started making nest and was scare chitless when you fired the truck up in the morning. Probably a mother looking for a place to give birth and wanted to find a warm place quick.
You ever see the picture of the snake that climbed on top of the engine of a civic to stay warm one night, well you could imagine the mess when the engine started in the morning, and it was weaved around the serpentine belt!! EWWWW