This has just started happening: when I first start the car (02 A6 3.0) and put it into reverse, in addition to the rear parking sensor beep (letting me know the system is working) there is a sustained beep for about 4 or 5 seconds that comes from somewhere in the right rear. This only happens when I first start the car - if I put it into reverse again, the beep doesn't happen.

Any hints as to what's going on?

Is park assist still working? If it isn't that long beep is a trouble indicator, something could be wrong with one of the sensors. You'll have to check the sensors.The beep happens only once when you start the car and put it in reverse.
There is a post by someone who did park assist as an add-on it might be helpful.
Let us know how you make out.

You were right - the system is not working. Thanks for your reponse - I'll post a question about probable causes and potential solutions. Do you have any thoughts about that?

Bad sensor. Check out an earlier post from Boston Driver, he suggests to look for error codes with the vag. You will still have to replace a sensor(s), $200 at the dealer.

While the sensor might have failed, more likely one of the sensors is dirty. I experienced the same thing....sometimes it worked fine, other times it just gave me a constant tone. No VAG COM errors on mine, but despite a car wash, the sensors were still a bit dirty. I keep a pack of Windex quick-wipes in the trunk, and find that a periodic wipe of the rear sensors clears that up.

Still, if you can find a VAG COM, just make sure it is not a blown sensor. I hope yours is just dirty...it costs nothing to clean them vs. $$$ to replace one.

Thanks. The car got quite dirty and although it's now been through the car wash, I'll try your tip about the wipes this weekend.

Good advice BD. I had the same problem I tried cleaning them, def cheaper, but it didn't help. I brought it to the dealer for warranty work asked them to check it out. They said sensor was damaged and needed to be replaced $200, it happened before I purchased the car. If we keep having these warm days maybe I'll try to fix it.

PS You have e-mail.