X = Response time of security company. In other words, how long it takes for the security company to get to the sight once alarm has been triggered. If the alarm is only triggered once the criminal is in the building, it is easy to get out before the security company responds. So the criminals don’t struggle to get out during X time.
But if the alarm is triggered outside the fence, he still has to get in and out, and his chances of escaping during X time are greatly reduced.
Studies show that when a criminal is detected at the fence, he will abandon his intention to commit the crime.