The alarm siren on my Avant started going off by itself without the alarm being triggered (i.e. no hazards flashing) and having read before this was a symptom of the siren packing up I set to and investigated.

I think on an Avant, if you don't have the Bose sound system with the sub in the rear quarter you could probably access the siren quickly and easily. However that is not the case on mine so to get to it:

1. Remove the back seat base (pull up at the front edge, it then unclips and can be removed). Audi say the clips need to be replaced but I have never broken one and they seem fine to me.
2. Remove the drivers side rear seat bolster, i.e. the bit of foam the seat belt runs over. To do this there is a bolt at the bottom and the large steel pin that the seat back locks onto needs to be removed.
3. Remove the tailgate slam trim, four torx head screws along the inside edge and pull it off.
4. Remove the two tie down points on the drivers side boot floor
5. Remove the boot floor (good pull backwards and it unclips)
6. Remove the small torx head screw which is in the slot where the boot cover blind clips in
7. Pull the side panel off. Sounds easy, you need to be brutal, there are a load of clips holding it in, but it will come off.
8. As you remove the panel, unclip the power lead to the 12v socket
9. Unbolt the Bose sub speaker, unclip the wires and remove it.
10. Remove the sounder which is in the bottom of the hole left behind.

There is a how to somewhere detailing cutting open the sounder, removing the expired cells and replacing them with rechargable AA's (the original type cells are no longer available). I cut mine open and the cells had leaked corroding and damaging the PCB, so new siren required. The siren has been superseeded and Audi claim a new modified bracket is required along with a new nut. Siren is about £140, bracket is about £10 and the nut is 30p (prices from TPS - YMMV). It transpires the bracket is no longer available, but they sent everything else. It turns out the only reason you cannot reuse the existing bracket is that the hole for the old siren is 6mm and the new one needs an 8mm hole, so that was £10 saved that didn't need to be spent and 30 seconds with a pillar drill to modify the bracket.

Put it all back together (swearing profusely getting the side panel back on - lots of clips to try and line up, it seems impossible but it will go). Job done.