I found some unexplained oil(?)/coolant/soot residue on and around my coolant expansion tank. My theory is that it is coolant that "steamed" out from underneath the cap and got dirty with exhaust soot. But it just seems like there is too much soot and it has an oily like feel to it. It has accumulated on the coolant expansion tank, under the cap lip and additional drops appear to have been sprayed/dripped on the surrounding arreas. Has anyone seen this before, know what happened, know why it happened, and how to prevent it from happening again?

It doesn't appear to emanate from, and I don't believe it is, but any chance it is associated with a malfunction of the PCV oil separator?