Christian, I recently arrived at my hangar, only to find a huge puddle of oil under my airplane. As usual with oil spills, it looked like a gallon on the floor, but was probably less than a cup total...
I pulled off the cowl and checked the engine for leaks by wiping it down everywhere I could reach. The only oil residue I found was on the side of the oil tank itself, and on the cowling directly below the tank. I got a local A&P to come look at it with me, just to be sure. We could not find any leaks. We ran it up (sans cowl) and there were no more leaks.
We finally determined (by process of elimination) that I must have spilled some oil when I was topping up the reservoir after my last flight before the "oil spill"... I had used a disposable paper funnel instead of my usual (and much smaller outlet) plastic one (lost when I moved to the new hangar). I suspect that while pouring the oil into the funnel, I inadvertently allowed the lip of the funnel to slip outside the oil filler, and that oil ran down the tank side, dripped onto the cowling, then onto the floor. Since topping off the oil was the last thing I did before turning out the lights and heading home, I just missed seeing the oil dripping onto the floor. (Of course, my drip pan was 2 inches aft of where the oil dripped off the cowling... Murphy's law in action.)
Just thought this might have bearing on your issue as well. Mine wasn't overfilled, but I did manage to overflow the side of the tank opening. Sigh...
