The alarm comes on when locked for no reason. No flashing lights just this annoying high pitched noise.

Have found what it is and have cured the problem by following this amazing link (good one for John-H)

www.tt-forum.co.uk/forum/viewtopic.php?t=73315&highlight=

It's the same for the B6 avant. Located in rear back drivers side access hatch in boot. Not so simple you can put your hand in and remove it, no!!!!!!! you have to take all the internal side panel off, the speaker sound box (large plastic thing with speaker in it) and the amplifier, unplug all electric plugs including the cigar lighter socket.

Finally you can get at the siren box, sealed unit with internal rechargable batteries.

Unfortunately, as link said, my batteries had leaked through to the cicuit board and damaged it beyond repair.

I phoned Audi for part, £84 plus VAT. two days wait.

As I would have to wait for the part, I decided to fit the new siren externally in the boot space, not hidden away, so adjusted the wiring and mounting bracket. That way I could put everything back in place ready. Took a bit of paitence but got there in the end, just leaving a wiring plug visible for this siren.

Anyway, long story, I have decided the locking, unlocking, internal sensor and LED on door all work without this siren, so decided not to bother with it at all.The only difference now, is the LED on door when locking, stays on constant for about 10 secs longer than before, then flashes as normal. I can live with that :)

I'm just wondering if anyone knows what the consequences of not having this siren are, apart from the obvious noise deterent (which was pathetic anyway) as at the moment every thing seems OK.