So the coolant level on my brand new car, a SS1600 (Sigma 275), is varying a lot from week to week. It was at Max when I picked it up, then when I checked the following week it was at Min. Topped it up, and then the week after it was way over Max, almost brimming, so took some out. Now this week it's back down way below Min again. It's all over the place ! Can't see any leaks, checking when cold. Is it meant to vary with heater use maybe ?