I've been experiencing coolant loss on my 1.6TDI A3 8P

Got no visible coolant leaks on the floor (engine undertray is off) and no white smoke coming from exhaust (idle/on throttle and moving)

Done a dye test that came back alarmingly quickly as a head gasket failure

But it's not doing the other usual head gasket failure symptoms - which is strange

Is there anything else that could be wrong that causing phantom coolant loss

How much coolant is it actually losing in say 200miles Chris? 

max to under minimum in under a week Gareth

Many thanks Chris - that’s obviously considerable. 
If you are absolutely sure there aren’t any leaks visible , externally, then you might consider HG issues, but I would carryout the following initial test:- 

Take the car for a run of at least 10 miles, then without switching the engine off, get an assistant to hold the revs. at a steady 2,000 rpm for 2 minutes while you observe the exhaust gasses. Report back.

Regards, 

Gareth.