It's activated/deactivated when you lock and unlock the doors. As you found out, it goes off for no apparent reason. Even with a "new" black box, you will still have that problem. Do a search here and you'll see that most just unplug the box and leave it that way. If you want an alarm, I suggest you pick an aftermarket one up.

__________________
99 ML320
94 SL600
92 SL500
95 E320 Cabriolet
87 560SEC
86 300SDL Grease Car
80 380SLC Euro

13 Fiat Abarth
02 Maserati Spyder Cambiocorsa
00 BMW Z3
90 Rolls Royce Silver Spur
80 Ferrari 308 GTSI
88 Jaguar XJS12 H&E Edition
99 Land Rover Discovery