I have a 2.8 Quattro that has a coolant leak issue as of late. I just replaced teh temp sensor and shortly after I noticed I had a coolant leak. I have checked the temp sensor and it isn't coming from there. It wont leak when the engine is cold. Once driving and the engine is warm it starts and will empty the recovery tank within a twenty minute drive.

Could this be the coolant flange on the back side of the engine?

 

Get the car running and warm and start looking for leaks. Could be a ripped/rotted hose, broken clamp, cracked reservoir (more common than you might think), or a line that popped off. Go for a drive, get it up to temp, then park it over cardboard and watch for leaks. Any smoke out the exhaust that would indicate you're burning coolant?