My MBs have always been a little oily in that area, but usually it's from a leak forward being blown back. On the M104 the most common leak spot is the front timing cover, right at the corner where it meets the head gasket. Of course, that tends to leave oil in the belly pan too, which you said was dry, but I would check there first.
Double-check your oil level. High levels had a tendency to cause the seal to leak:
http://www.peachparts.com/shopforum/tech-help/122696-104-rear-main-oil-leak.html