Not sure if you've got a new car - see recent posts under vehicle electrics - 3 of us had similar problem, which ended up being problem with control unit requiring replacement part - luckily once one person found out what problem was having had various sensors etc replaced unnecessarily and could then tell garage what to look at first. Other than that, could be doors or apparently leaving air vents open can cause motion alarm to set off - may well need trip to garage, I don't think these things are ever obvious.
