Depends.... When hot , there are oil fumes that drifting to air and but when engine stop and cools down , oil starts to concentrate on the surface and creates the leak.
For example , my valve seals are very worn and i have excessive oil fumes and since i have an open system that doesnt recycle them to the engine i had to guide them to go down the car floor to avoid having them inside the cabin. This way i have dirty and oily oil sump and when i stop the engine i have lots of oil drop underneath the car.
Sprint 1.7 '85