2 years ago I was descending into Milan Linate at night from the North having just crossed the Alps in the airway at FL150. As I passed 5000 ft I began to smell burning and was rather concerned about the possibility of an engine fire, I couldn't figure it out - all the gauges were fine and the engine sounded perfect.
After I landed I realised that the topography of the Po valley area is a real smog bowl and traps all the Vespa exhausts and pizza smoke created by the Northern Italians.
This is what my burning smell was.
Sometimes it is outside, even at altitude,
SB