Coolant leak

[Danton J.A. Cardoso]

Patient is an A4, 2.8 quattro five speed with 143,xxx. Timing belt with water pump was replaced roughly 3K ago. 

Drove yesterday with heat on in various driving situations. Outside temp  low 40's, temp gauge in middle.  Car sat out overnight, temp in mid to low 30's. Started car this morning temp light flashed and beeped. Shut down car, coolant tank almost drained. No visible leaks. 

What could this be?

Prost! 

Danton J. A. Cardoso