Oil stem seals mate, I have the same problem....I bought my current car knowing they needed to be done and my previous GZ Aerotop showed the problem up when I went full decat.
Once the car is warmed up and say I am sitting in traffic, small blip on the accelerator and there is a puff off smoke. Hard acceleration and all is clear, its only on startup first thing, sometimes small puff of smoke, sometimes a cloud and the rest of the time on stop/start in traffic.
You will find the reason in the forum threads, just got to decide if it is something you wan't to get fixed or not !
I have to admit it is tempting for me, I do a short journey to work, I hate leaving smoke !
However got to balance that up against approx £700 to fix it......