W220 oil leak!

I have a W220 S230 which seems to be losing oil after long journeys. I check the oil level with the car after after such journeys and it states to add

I've check the engine area and there is a oil stains by the power steering pump. RHD of the engine if are you looking from front of car. Just had the car serviced and they cant really locate the leak so asked them to clean up the mess. I will check again after a few days to see if I can spot the cause of the leak.

Does anyone know of any possible reasons for an oil leak on such a car?

Thanks!

Jim