2002 passat allarm going off

i have a 2002 tdi 130 passat and the alarm keeps going off....have checked door open sensors and they all work inc' boot. the only thing i can find wrong is that the indicators dont always flash flash when i lock the car.
the alarm only seems to go off when the indicators dont flash when locking the car, when they do it seems to be ok....someone pls help as this problem seems to be getting worse

I had this problem recently. The breakdown man thought it might be a problem with either the boot (trunk) or bonnet (hood) switches. Took it to local VW dealer who said alarm connections (located somewhere under the front passenger footwell had become corroded because water had leaked in through a leak in the pollen filter ! Expensive to fix - 6 hours labour charged !