i wouldnt worry about it to much, its probably a bit of engine oil, but it could have been there for years.. as for the sandy bits, thats probably just grit and dust from the road thats stuck to the oil over time
a little bit of oil can go a long way! if you had a leak, there would more than likely be drip warks on the ground under it.. a good degreaser and old rag will get it off i'd imagine too. hope this helps
unless your talking about the under seal..