Theft alarm sounds when I lock the car

Hi All,

Next question about my two-week-old (to me) 2006 06 Octavia 1.9 TDi PD Ambiente estate.

Last night, in the howling rain and gale, the alarm went off for no apparent reason while the car was parked outside the house. I assumed that pressure changes or a gust of wind had set it off. However, it now seems to go off each time we lock the car. The alarm stops if you unlock it and then lock it again. (Always using the remote.)

Any ideas why? Is there a battery somewhere which needs changing? Might some water have got into the electrics? Has anyone else had this problem?

Thanks in advance,

scandalxk