Had another search and came across the following thread:
https://www.audiworld.com/forums/a8-...tions-1748666/
Post #6 links to three images. The first and third I can find. Is the second one the one you mention?
Never had an issue with that pipe before and the clip was as tight as a tight thing.
Also one about the link above. In post #4 the person asks about running the engine. Is it safe to do this for a few seconds, and will it really help to drain more of the coolant if the drain plugs are out and radiator disconnected?
It was 2 degrees when I was out earlier, so a lot warmer than this morning, but I'm going to get something to eat and then go out again and see if I can drain more from these plugs.
__________________
2001 A8 D2 FL 3.7 Q - LPG Conversion, Engine: AKC, Gearbox:FBB, Colour:LY5X Aqua Blue pearl effect, Interior:WJ light beige/dark beige valcona leather with burr walnut inserts
My DIY jobs -
Replaced alarm horn,
Replaced Coolant Temp sensor,
Updated RNS-D firmware,
Installed Reversing Camera and Digital TV,
Dashcam Installation,
Retrofitted Rear Blinds,
Auto-Dimming Rear View Mirror,
Chrome Boot Struts
Sold! 
2008 S8 D3 5.2 V10 - Engine: BSM, Gearbox:KLW, Colour:LY7F Suzuka Grey Pearl Effect, Interior:Black/Black with Silver/Black valcona leather with carbon atlas inserts, Bang & Olufsen, Heated windscreen, Soft-close doors, TV, DAB, AMI, Quad Zone, Auto Boot, Auto dim Mirrors, Keyless entry, PDC with rearview camera, AFS II Headlights
My DIY jobs -
Fan Jet Windscreen Washers,
Retrofitted Electric Side Blinds