Oil in cylinder lower surface

Hello guys!

Today I have noticed some oil in the lower area of the left back cylinder when I was checking the spark plugs during my pre flight inspection. It was just some oil in the area with no visible leak on the hangar floor or no oil over other areas of the engine or the engine cowl. I need some advice about how to diagnose the origin and how to proceed... Today I checked the oil level, refill it up to max level, I went flying with oil pressure being fine during the flight and checked again oil level after the flight. How can I proceed to determine the cause of the leak?

Thanks a lot!!