About two months ago my A6 developed a water leak. About one (1) coolant resevoir a week, almost to the day. I don't see any evidence of a leak however, no puddles after parking, no dripping while idling. Quite baffling.
Any ideas?

 

What engine is in your A6?

Do you see any white smoke from your exhaust? Check below the intake manifold and see if there is any coolant pooling up in the valley. You may have a leaking secondary water pump.

 

On my son's A6 2.7, it had been mysteriously loosing coolant also, with no signs of drips below engine. Difficult to see below intake manifold to region of 2ndary coolant pump, but there are some angles that give you a glimpse. Changed coolant pump & inlet/outlet hoses & problem solved. This not the only cause for coolant loss, but a common one, if you have the 2ndary pump.

 

It's a 2.7T - I'll check the secondary water pump.

 

no leaks around the 2ndry coolant pump.

 

Perhaps pressure test the system and listen / look for leaks. Although uncommon, I would also look into whether or not your oil looks like a milkshake. If it does, then you are getting coolant into the crank case and the headgasket could be the issue.