Could this be a DPF regeneration?

I do a regular 280 mile journey of which over 200 are on the motorway (typical lane 3 speeds). My last stop was about 20 miles before the end of the journey, and when I stopped at my destination the fan came on. I've never noticed a regeneration before in 50,000 miles and 5 years of ownership, but could this be what it is, and what's the best thing to do now I've interrupted it?

Forgot to say that final minute or so is crawling at idle in 2nd gear. (Allows turbo to simmer whilst on a dirt track).

Thanks for the info, but I've read on other forums that as a result of the burn off, the fan often comes on once the car is parked to dissipate the heat built up. I've noticed this on a MY11 TT with a CR170 engine, but obviously my A4 is a fair bit older, so I'm not sure if the mileage based regen (every 300 miles?) applies to the PD engine. I'm just wondering whether the regen has finished, or if it's been interrupted in which case I probably need to take it for a longer drive.

Other than a stutter before the injectors were replaced, the car never drives noticeably differently, so I'm just wondering if I've just coincided with a mileage based regen (if that applies).

This was the first time I've noticed it, and the car hadn't been driven hard - just a long distance with a couple of stops. Ambient temp was about 18 degC and the fan stayed on for about 30 seconds.