I saw these the other day on the ground in my back yard. The weather was damp and cloudy - it even set off the flash when I took a picture.
From what I have read they must have been mating (they had gone when I looked a couple of hours later). However, I thought they normally paired hanging from a branch or something, not flat on the ground. Is this unusual? What species of slug are they? It was interesting that they were different shades of brown, one much lighter than teh other.