Just bought myself a 2015 A6 3.0TDI Quattro 272ps Avant… absolutely love it. 
Car has 70k miles and a very comprehensive service history, including two S Tronic gearbox services (thankfully!!). I noticed however in cold start (for example first one in the morning) a puff of blue smoke, that isn’t there on warm starts or when driving on hard acceleration (done this with someone driving behind me). Car drives grand. Just curious as to why this is happening, should I be worried and what or where should I be looking? Obviously have some time on my side with the dealership I bought her from to cover under the legal 30 day warranty, so want to get to this now rather than later. Thank you advance everyone. 

Could be valve guide seals weeping a bit overnight. Try lifting your foot off the accelerator letting the wheels drive the car on over run, then accelerate fairly briskly if they is blue smoke it could be the valve guides.

Thanks Anton