I'm riding home yesterday evening and its pretty warm out. I get a wiff of something burning. Not sure what it is. Maybe its the car in front of me... Nope. Ok, maybe the bike is somehow warmer than normal, because its been cold out lately? Doesn't make sense, but I'm just trying to figure it out.
Get home and I still smell something... Well we'll see how it is tomorrow.
Today I fire the bike up and within a few minutes I smell something burning. Almost like wood smoke - not oil or something like that. I start looking around. Ah...
An oak leaf stuck between your pipes and the frame - yeah, it will smolder and put off a smell.