If i let my water run low i find a bit of gunk on the cap.
Which cap, if is is the oil cap, maybe nothing to worry about. Condensated oil fumes can make a mess on the filter at this time of year.
Where is the water going is what I would be looking
at, it you had a big head gasket problem the oil in the sump will be contaminated. Does the car missfire when you first start it?.
Steve.