I consulted a friend today, he works exclusively with VW / Audi products. He had the following comments.
If it were the later 2.0 bi turbo the news could be very bleak. The CFCA 2.0 180hp engines have had a really bad reputation of high oil use and engine failure, all to do with corrosion in the EGR assembly, apparently.
The 2.5 though is regarded, by him, as "bomb proof." If there is high oil use, and you are using the correct oil and can't find oil leaking out anywhere, then it could be one of the following,
Oil can leak into the power steering system, but if you've lost enough to bring on the oil light this should have been obvious, with oil coming out of the steering reservoir.
Oil can also leak from the oil cooler into the water system, but it's usually the opposite symptom of water in the oil. Again he said that if this had happened it should also be obvious with oil in the header tank.
A blocked turbo oil return hose or turbo seals. Oil will usually be found inside the intake hoses. This is important to check because excessive oil could cause the motor to "Runaway" basically run on the oil rather than diesel.
And finally - it could be a worn motor, if all other routes look fine have a compression test undertaken.
OR - The Garage just didn't put enough oil in