I was having an issue with car temp gauge rising to right under the red. I replaced thermostat temperature sensor and radiator fan switch which seems to have corrected the overheating issue however now I'm having a problem with coolant loss at the reservoir. Before changing thermostat coolant was rising when car was hot above the full line then once the car cooled it would go back to full now what's happening is whatever level I fill the coolant reservoir with when I come back from driving it stays there but by the time the car cools or sits for an hour or two the coolant is gone. There are no visible leaks whether from thermostat housing water pump radiator tubes or radiator. Car is blowing hot heat with no problem. Im very confused because before changing the thermostat I was having coolant leak but not as much or nearly as much

 

I think I would start the car, open the hood and then observe. I'd look to see if the tank overflows and I'd also check the radiator cap as well when things heat up and pressurize,

 

Oh, just changed a failed radiator cap. It was the original and did not seal right and was always using just a bit every month. I also did a pressure check on it with a rental from autozone and a pressure check on the coolant system. The cap did not hold 15 psi. Dropped like a stone while coolant system held at 15 psi. This was on my suburban truck.