Yep , what you describe sounds like a regen. I would have expected it earlier but that said if you are driving to get a mpg of 40.6 over 670 miles then the need to regen could be much less.
Once the car has been fully run-in I would expect more like 4miles of regen per 100miles but again , that depends on how you drive. More spirited driving will promote more regens in my opinion although its still a very much unknown 'event'.
Clear indicators of a regen are reduced economy , reduced power and increased heat from the exhaust (mostly only noticed if you park up after or soon after a regen when you will hear the exhaust 'ticking' as it cools). Some also say they can smell it but I have yet to notice this personally. These indicators could also be a sign of the car not getting the chance to regen and being on the way to a 'limp mode' crawl to the nearest Alfa dealer for a manual regen (due to the exhaust getting slowly blocked).