Just a thought. The wind and air will spread a very minute of
oil or coolant all over and make it really hard to find where it may come from. Have you tried wiping everything completely clean and then just running the car until everything is up to temperature and watching to see if you can see where it may come from? I had a tiny pin hole in a line once that I looked for and couldn't find so I did that and I also added that stuff that shows up under a black light. Then when I ran the car I spotted it almost immediately and it wasn't close to where the darn film was always showing up. It was about 3 inches in front of it and the wind was blowing it back and spreading it.
Ron