The RS6 coolant system wraps itself around most of the engine compartment and can be prone to a variety of leaks; many due to the extensive plumbing to and from the Auxiliary Radiators located behind the Intercoolers. Aside from a catastrophic head gasket failure, there are only so many locations coolant will leak. Here is a top 10 list to cover most everything. Also be observant of the oil and transmission coolers located in front of the main radiator.
1. Auxiliary Radiators located in front of each wheel, behind each Intercooler.
2. Radiator that is located in the front clip
3. Coolant return pipe on back of heads - will drip fluid just behind drive axles on either side.
4. Turbo lines - will drip fluid just behind drive axles on either side.
6. Temperature sensor - will drip fluid just behind drivers side axle. Easy to see from above.
7. Recirculation pump located on underside of drivers side frame rail.
8. Heater core - would smell coolant inside car.
9. Overflow tank - will drip fluid into firewall area, and it could be smelled sometimes as it hits the exhaust.
10. After run pump under intake manifold - coolant will be hard to see coming from here and make it's way either towards the front of the engine or rear, and not produce consistent puddles (and possibly two at the same time).
11. Plastic coolant pipe bridging the block and the primary oil cooler assembly. This is commonly replaced with an upgraded Aluminium piece crafted by Bufkin Engineering. More of this is discussed on the Maintenance page.
Hoses should all be pretty obvious to tell if they are leaking. Some items are moderately straight forward to repair, while others are extremely difficult to near impossible without significant mechanical undertaking. Some users have also removed the extra Auxiliary Radiators that were supplied in all warm weather destined European cars, and all models send to the United States.