could be any number of things. the 2 that come to mind are a micro switch in a door/trunk has gone bad.

has your driver side floor board been wet? if it has, might need to replace some wires under your carpet due to corrosion from water damage.

 

See my thread here for some ideas. I am expderiencing the same problem. Get a scan of your car and it might help narrow down the possibilities.

http://www.passatworld.com/forums/showthread.php?t=281259

good luck... I'm off to investigate.

 

There is an easier and definitive way. Just ask the CCM which lock is causing the problem! Follow instructions below. The CCM tracks the code for the lock that has caused the alarm to go off the previous four times. Look at these codes and they will tell you definitively where your problem is.

Go into:

[Select]
[46 - Cent. Conv.]
[Meas Blocks - 08]
Enter group 015
[Go!]

Fields 1 through 4 should each show the cause of the last 4 alarms (first field is most recent, etc). The meaning of the values in each field are:

Rear lid/tailgate contact switch
1

Rear right rotary latch switch
2

Rear left rotary latch switch
4

Front passenger rotary latch switch
8

Ignition
16

Interior monitoring
32

Hood contact switch
64

Driver's rotary latch switch
128

No alarm
255

 

I had the same problem. Ever since I replaced the Microswitch it has not went off since and that was a year and a half ago.

Here is a write up (done on my 98 b5)
http://www.passatworld.com/forums/showthread.php?t=96941
Scroll down on that post I posted pics with the instructions.

 

I have this problem its something in the drivers door latch and drivers rear door latch.. that the alarm won't sense when the door is open.. so if I unlock it and don't start it quickly it re locks.. and can't tell that the door is open... the light doesn't even come on under the door... I don't care anymore its been like that since I got the car... I've just learned to live with it... its not annoying to me...