It's condensation.
Mine also started smoking on warm-up.
Just as the weather got cold!
Your breath causes a bit of steam, but the exhaust gasses are a lot hotter so they produce a lot more, but same process.
As the exhaust system warms up the problem goes away...
Unless there's something seriously wrong with our engines. Thanks for starting that paranoia, Geoff!
I always ride it as if it's about to break anyway!