It can also be your heater control valve. There is a hose that exits the head drivers side toward the fire wall. It goes to a heater control valve and then another hose feeds it into fire wall. You could have coolant evaporating from that spot. I have had the problem before, actually caused me to have an overheat and subsequent head gasket issue. With me I did the head gasket job and noticed I was still losing coolant but could not find leak so I was thinking it was going in the motor. Turns out is was evaporating through hcv. I learned that when the car started to overheat again i pulled over right away popped the hood and seen all the evaporation leaving the hcv. Changed out part and am holding steady now knock on wood.