Practical Joke?
Maybe youf Girlfriend went down to your local auto supply store and kinda,, dumped 1 gallon of oil under your truck just to scare you, LOL
On the driver side of the truck there are a couple "dark fluid" containers.
Power Steering
Brake Fluid
Engine Oil
it could be any one of those.
I've been reading about several Power Steering boxes leaking lately. You might want to check that.
I don't know how the brake fluid would leak,, besides a near failure of the fluid reserve bottle. (that bottle thing you pour the brake fluid in

What is the Correct term for it? )
Since the temperature "up there"

changed it could be from fluid expanding or a gasket letting go of it's seal, bolts/nuts loosening up,, etc. etc.
Also, I don't know if you are the type to do this,, but around here on the border I see it ALOT!!
People seem to like putting there Antifreeze, Power steering, Transmission fluid, brake fluid, and motor oil underneath their hood for storage. If you or somebody happened to leave a gallon bottle of oil under the truck it could have turned over and/or leaked out when the truck was stopped,, or soon before/after.
I bring up these possibilities, not because I think you would do them, but because you say there is oil on the ground, and the Engine still has all the oil it is supposed to.
Hope these help,
MerrickNJr