How full in a cold state should the tank for the radiator fluid be? Had the tensioner job done to it in march and at the same time had the water pump replaced and that system topped off now this week seeing low coolant warnings popping. It is on when you first start the car when it is cold, been sitting all night. Start car, warning is on. Turn it off right away and back on and warning goes away and at this point is not coming back on until it sits all night again in what now 35 degree nights. Car sits in a detached garage.
I suspect this damn car is burning off the coolant as I see NO leaks anywhere.

 

After filling the tank completely and assuming a fully functional cooling system the tank should stabilize down about an inch or two from the top when cold. But not low enough to set a low coolant warning. Hot vs. cold will make a significant difference in that little tank. Too full and it will just overflow into the secondary overflow bottle, not to be returned.

If you have a reoccurring low level there has got to be a small leak somewhere. Look around the base of the thermostat tower.

I believe that I damaged my tank when replacing the tensioners. With the upper hoses removed, the tank will swing out of the way of the valve cover attached to the bottom hose to the heater system. Left loose it will also swing back and strike its metal bracket on the side below the seam line. Mine had a very small pin hole just about where it would have hit the bracket. The hole was invisible and there was no mark indicating damage but under pressure from a tester produced a very fine mist.

Installed a used tank which developed a small leak in exactly the same place. Rather than buy another I cleaned, sanded and applied JB Weld which has held for 6 months now.

 

The front of the motor, around the Thermo Tower, looks clean as can be for a 75k mile car. No look of old leakage from coolant. I will keep an I on the tank. When cold it was full JUST to the top of the opening at the bottom of the tank. Assuming it was topped off correctly after water pump was replaced it has taken 6 months to leak to this level. I have never smelled anything coolant wise either.

 

It took awhile for the air bubbles to make it out of my system after replacing the tank, just keep topping off once in awhile, and feel free to bring the level up higher than the bottom of opening at the bottom... more than likely that's why.

 

After the tensioner repair in which I had the water pump replaced they did say that they "tried" there best to burp the system of air. Maybe it has done what you said. I will keep an eye on it, see what happens.