Since we bought a house in 2009 (then 5 years old), I have noticed a slow accumulation of (what i'm assuming is) oil sitting in the grass and on the stepping stones on the outside of our house, on the other side of the wall from the water heater (in our garage) where there is a small pipe that constantly drips water. The area that is oily never dries and now encompasses a 5x3 ft area. The stepping stones in that area always have a thick black rainbow sheen to them. We are stumped. Any ideas about what is causing this?