Mauricio,
The sensor on your box is what Conion dubbed a "burglar alarm". This is a common feature on some of their larger boxes. There is a button which I believe is above the power button on your model that will turn on the alarm. When the alarm is set and a proximity barrier is broken the alarm will set off a rather high pitch alarm to indicate that someone "unauthorized" is too close to it's location. Once set there's usually is a 30 second to 1 minute lag time to when it becomes active. This feature is sometimes finicky and with time, the alarm can fail to activate. Members here have had success in repairing this feature on their boxes. If it's not working search through the restoration forum to see if anyone has documented this repair before.
Good luck,
Jerry