the guys at the garage went through their records, and told me over the phone it cannot be anything serious. The car was fully serviced less than a month ago. Now, it will be a 2-minute job to reset the indicator.

So, what you say makes a lot of sense. They changed the oil, but forgot to reset the indicator.

And I was fancying some sophisticated self-diagnostic system, which would measure the performace at 72 different points, and store data about any malfunction. Now I suspect it is a simple counter, which comes up every 12,000 Kms or every 6 months, or whatever the service schedule is. An anti-climax with a happy ending.