Audi S4 Coolant Leak. It will be fine for like a week didn't go. Looks like you were on to something.
It’s because the eggheads at vw/audi designed the thermostat to be plastic, and after hundreds of cycles of heating/cooling the plastic cracks and starts leaking coolant. Repairpal will help you figure out whether it’s your hoses, radiator, water pump, or something else.