In reading an extended discussion on regeneration of the DPF it was mentioned that one sign of an interrupted regen was if the fans continued to run after the ignition was switched off. 

My question is .... Is that the only context in which they might continue to run? This has happened to me a couple of times, but in neither case would I have suspected a regen. If I again turn the ignition back in and off again they stop. Current car is 2017 Yeti 150 DSG. 

The same thing happened a few times with my previous car - Octy VRS CR diesel. I'm sure I read somewhere way back that this could also be down to air conditioning. 

Any views?

That will be down to an interrupted DPF regen.
To do a regen the temperatures are raised a LOT so if the engine is switched off the fans run to cool things down, it's best to leave it at that and not try to stop them.

Urrell

are you saying that this is always and only as a result of an interrupted regen?

Cooling fans can run on to do the cooling, of the coolant in the radiator to allow the water pump that keeps pumping to cool the oil once you have stopped.

It does not have to be because of a regen.

The same can happen with petrol engines that do not have regens.

Fans ran on long before dpfs appeared. 

This time of the year fans running when switching off is an interrupted regen unless the car has been working extremely hard towing or something like that.
When in a regen the tickover is raised to about 1000rpm with a gruff exhaust note.

That can be very true if you are in the UK in winter.

If people ask a question and do not say where they are you do not know the location at the left hand side if on a phone.

Someone asked today about a coolant leak and had a reply posted by a member about the cold winter weather and coolant loss and the OP is in Australia.

I'll take the confirmations that cans can run on for a variety of reasons that predate DPF - so thanks guys. Also I won't take any proactive steps to stop it when it happens. 

Mine will do region after 5 miles from a cold start. More frequent since the VW software 'fix'. As noted by Urrell, faster tickover and, in my case, a whiff of burning diesel

When my 170 regenerated, the idle speed would increase from approx 850rpm to 1000rpm. The gear shift indicator would also prompt to change down gears to keep the revs up. If you don't have either of these happening before you turn off the engine, I doubt it it is regenerating. I found the VW fix made no difference to the frequency of regenerating.

The 150cr engine in the Octavia does not prompt you to drive in a lower gear while it's doing a regen in my experience. My experience being running vag dpf app while driving. But yes, it does cause the car to idle at about 1000rpm, and prevents stop start from happening.

The OP has a TDI with SCR (ad-blue), they do behave a bit differently.