Just following up on this - I have my car up on stands today (because there is some weird shunt in the drivetrain when pulling away and I wanted to see if any bolts were loose or missing.....anyway) and I noticed that where the pipes join the back section and are secured with compression clamps around them, on one side there is a liberal coating of soot right next to the joint. I would post a picture but that seems like a tricky job here. The other side was much cleaner (the whole underneath of the car is pretty clean due to lack of miles and only dry miles)
So perhaps the sporadic smells coincide with gas/emissions escaping at the joint - perhaps under high load or maybe high vibration. Anyway - I thought I would share that observation. I've tightened the clamp up
