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A couple of thoughts. I eventually learned to always check under the hood after a service - mainly to make sure the dipstick is back in place (I once found my dipstick lying across the top of the engine) and the filler cap is back on (I know more than one person that had oil blow out of an uncovered fill hole because Jo-Jo forgot to put it back) and once I get home I check the level right away.
It was my C5 that convinced me to start doing my own changes. After the first oil change at the dealer I noticed some oil drops in my garage and reached under to find the filter was not even fully hand tight. I never let anyone touch the car again for anything other than warranty work.