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That is your oil filter housing cap. Since it is upside down, a mess is often made when changing the oil—the cap is full of oil. Oil also drips out of the housing when the cap is off. It often gets onto the aluminum plate and then drips on the ground later. You can take the time to clean up (I do) but many mechanics would rather move onto the next money making job.
Good news is that very recently BMW came out with a cap that has a drain plug on it. You could buy one and ask your dealer to install it next oil change. If the mechanic will take the time to pull the cap drain plug and drain the filter housing while draining the oil pan, your car should be cleaner afterwards. But the mechanic might not care to spend that extra time.
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