I understand the problem, it's just a good thing they paid more attention to getting the drain plug tight than they did to the belly pan. Have you checked to see that the drain plug is tight?
I can see where it would be tough to make a living in a shop full of techs, as opposed to high school kids at the Jiffy Lube, by doing oil changes. If it's a tech doing the work though, hopefully he's watching for other things. The main reason I do my own oil changes is so that I can get a look under the car to see what may be falling off.
'84 Euro 240D, 523K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in