Are you always bleeding water off and never filling it up?
The fluid must come from somewhere?
I have seen similar on my car, but I put that down to the heater being above the max mark on the exp tank, I would fill the car cold and bleed leaving air in the exp tank, do a run and then find the exp tank brimmed. Release some coolant whilst hot and then carry on. When cold I would have not enough coolant, refill and bleed from the heater with the front in the air and the cycle went on.
In the end I left well alone when it was hot, and then corrected the level when cold after a track day. All has been fine since. I think they need a bit of air in the heater otherwise the air wants to go to the top, which on mine at least appears to be the heater matrix and hence the exp tank fills up. The heater still works and the temps are fine
I have a K 1.6 SS, c/w dry sump, fresh air heater, flat exp tank radtec rad and boggo stat.
Doug