X = Response time of the security company. In other words, how long it takes for the security company to get to the site once the 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. It will be easier for the criminal to escape during X time.
However, if the alarm is triggered outside the fence, the criminal still needs to get in and out, and his chances of escaping during X time is greatly reduced. Studies show that when a criminal is detected at the fence, he will most likely abandon his intention to commit the crime.