Car broken into

So our car got broken into in the parking of the neighbourhood Carrefour in Ixelles, while we were shopping inside. There were a lot of other cars/people around as it was around 06.30 pm. However there was no sign of forced entry-no window broken etc. We have had smashed windows in the past but does anyone know how does this 'in-forced' break-in works-without triggering an alarm? Would be good to know so that we can avoid anything that we are doing wrong here.

Luckily there was nothing inside the car worth stealing so they just took sunglasses(!!) -