Check to see if you caught a plastic bag anywhere near the exhaust system underneath the car or on the mufflers and you still have melted plastic on any of the hot parts. This would smell the most when you have the car parked in the garage after driving it. This would match your description of "melting".
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The Coupe: 2016 M4 | Sakhir Orange | Black Full Merino Leather | CF Trim | M-DCT | More | ED 5/13/16
The Sedan: 2018 M3 | San Marino Blue | Black Full Merino Leather | CF Trim | M-DCT | ZCP | ED 7/18/18
The Roadster: 2006 Z4 | Interlagos Blue | Black Extended Nappa Leather | Carbon Leather Trim | Purchased 7/19/12
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