ALARM NOT WORKONG

someone tried to stole my vw polo vivo,broke the door lock,open the door,open the fuse box and took a fuse out all this without the alarm going off??it takes the alarm about 15 seconds to go off after the door is open.wich give you enough time to open the fuse box and take the fuse out that controls the alarm?this i cant understand the alarm is suppose the go off the moment the door is opened??