Pretty much every day there is a police check somewhere along the way to school. Either on one of the many roundabouts on Carretera de Vallvidrera or on the big roundabout where Carretera de Rubi meets the Autopista de Montserrat.
Now we finally know what they are looking for. Luckily my wife managed to sweet-talk her way out of a ticket.
Once your car is more than four years old (and every two years after that), you are required to do a government revision of your car to ensure that everything is in working order. Once you complete this, they put a sticker on your windshield.
Basically the police is looking for any car that is more than 4 years old that doesn't have a sticker.