Mysterious beeping upon start-up

[nussaj]

I have a 2002 A4 Avant, and recently (last few months) every once in a while upon start-up there is a mysterious beeping (three times) that seems to come from the back of the car. It doesn't happen all of the time, and I can't seem to find a pattern. Any ideas?

[xmay4444]

There is a sticky thread called 'Three beeps from 1.8T' at the top of the page