I would imagine that your car has been attempting a regeneration during the drive and has only partially completed it.The fan will then come on, even after switching off and getting out of the car, to remove the excess temperature that it has been creating so as to burn off the soot within the DPF.
My car tried 3 times to do a regen last weekend but, the journeys we were doingwere 'urban' (Dunfermline to Dalgety Bay)so we never fully achieved it. Every time we stopped the fan roared into operation, sounding like a hovercraft, for about 30 seconds, after which it stops. We just left it as my wife's commute from Dunfermline to Stirling generally keeps the car happy with its regen process.
No DPF lights should be seen unless there is a problem, so don't worry about not having any indication on your dash.
You could always add a bottle of Wynns DPF Cleaner to your next tank of fuel to assist with the burn-off process. It's £9 from Halfrauds or cheaper on Ebay but, then you have a wait (plus postage/delivery etc).
There is a huge thread on DPF issues but, all modern diesels have them and no car is without some problem or other. If you are doing 'normal' driving and not just townie stuff you should be okay - After all, there seems to be a sh*t load of Antaras around these parts, so they must be doing something right!
Hope that helps.
Edited by: Lazarus
Antara 163PS: Bluefin Remapped to 210PS