Since the leak is from an obscure area you might consider this. Get a mechanic's mirror and a fairly bright trouble light and check this in the dark; either in the drive at night or in an enclosed garage.
Sometimes the contrast is better in the dark in my opinion anyway.
You could also try one of the fluorescent dye/light kits sold at many car parts stores. It's not expensive. Add the dye to the engine oil, run it for a while, and then (in the dark preferably) shine the light in there and see what's glowing.
Hope that helps.