An acquaintance in New England had an unsatisfactory alarm system in his house. After months or years of malfunctions and failed repairs, he asked the alarm company the question, How much would you charge for just the "Protected by" sign? Their answer: the same as for the system.
Most houses in our neighborhood have alarm systems. Their use will not necessarily prevent burglaries, but it may limit the time the burglars have to work; a woman across the street neglected to set hers when she ran an errand a couple of years ago, and the burglars had time to ransack the house thoroughly, and drink some of the household's wine.
We have not so far been burglarized, but the alarms do go off on their own now and then. For example, the alarm sounded in the small hours this morning, showing that the back window in the living room was ajar. In fact, the back window was secured. I wondered whether the wind might have jostled it a bit, but that window is behind a tightly fitted storm window.
The alarm I remember best occurred our first winter in the house, also in the small hours. A steel pot fell from the drying rack, the noise of its landing setting off the kitchen glass-break detector. Not much else would have got me out of a warm bed at that hour, and almost nothing else would have got me downstairs that fast. Wide awake, we have set off the glass-break detectors. I did it once by hammering down the lid on a paint can while the system was still armed; my wife did it more recently by closing a drawer full of kitchen utensils.
The alarm our neighbors remember best occurred when we were out of town--of course also in the small hours. As I recall it, they were treated to at least 45 minutes of noise over a couple of hours, and the police came. We returned to town with apologies and chocolate for those nearest the noise.