This problem occurs because the mechanic is unaware there is a washer there. When the plug is removed the washer initially sticks to the block. As the coolant gushes out it frees the washer and deposits it in the container the coolant is being collected in, where it is subsequently dumped. Then the "where is the seal" questions arise.
I think a quick call to your nearest Mercedes-Benz dealer should solve the problem. He should also be able to get you a seal for a quarter or two. Good Luck, and I hope this helps, Jim
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)