Ya' know...I really like my dealer, very good customer service and they run on time usually and I always have access to a loaner car. But sometimes I think the techs in the back miss the obvious solutions.
I had the hose from the upper tank to the radiator replaced but the level was still going down and the smell was still there. I thought to replace the fairly new radiator cap myself since it costs me about $5. I think the problem is now solved. Yesterday there was no smell at shutdown and the coolant level stayed the same.
I'm thinking about giving the service advisor a call as constrcutive feedback. This has happened once before and it involved the radiator cap again.

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dtf
1994 E320 Wagon (Died @ 308,669 miles)
1995 E300 Diesel (228,000)
1999 E300 Turbodiesel ( died @ 255,000)
2006 Toyota Tundra SR5 AC 4X4 (115,000 miles) rusted frame - sold to chop shop
2011 Audi A4 Avant (165,000 miles) Seized engine - donated to Salvation Army
BMW 330 xi 6 speed manual (175,034 miles)
2014 E350 4Matic Wagon 128,000 miles
2018 Dodge Ram 21,000 miles