Are you sure its not the DPF regeneration:
To maintain performance a DPF has to be emptied regularly. This is usually done passively in a process called 'regeneration': when the exhaust temperature's high enough, on motorways or fast A-roads. The collected soot is burnt off, leaving only a tiny ash residue. This can give off a smell during the process.
https://www.youtube.com/watch?v=aMKpo74P6SE