I think these are similar, but not exactly the same as motion detectors.
These detect not only inertia, but vibrations and shocks as well.

Many cars here have them, so if you are drunk and stumble onto a car (never happened to me obviously) the alarm goes off.
"a few systems have a shock sensor which will trigger upon a significant impact to the vehicle's body."
http://en.wikipedia.org/wiki/Car_alarm

I think some sensors/detectors serve both purposes though, and I think some people use them interchangeably, although it isn't strictly accurate.

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Note added at 42 mins (2012-02-21 16:12:48 GMT)
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"Shock detectors are extremely useful perimeter protection devices. They will detect an attempted intrusion before entry to the premises. They can also be left alarmed while the premises are occupied -allowing free movement within the premises while protecting external doors and windows."
(From the links I already provided)

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Note added at 43 mins (2012-02-21 16:13:37 GMT)
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Sensors/Detectors seem to be completely interchangeable. Sensors is definitely the more common collocation out of the two.

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Note added at 47 mins (2012-02-21 16:18:22 GMT)
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Also: with sensors/detectors there's little consistency. In one of the links I provided (http://www.diy-alarms.co.uk/catalog/index.php?cPath=142), the title is "Shock sensors" but then it goes on and talks about "shock detectors" on the line directly underneath.