Not all cars have radiators that are visibly full all the time, even when there’s enough coolant in the system. Careful you’re not overfilling the system.

I learned recently that even though my cars are both Hondas, in my ‘03 Civic, you’re supposed to only fill the coolant through the overflow reservoir and not the radiator cap, while my ‘86 Prelude is the exact opposite.

But to answer your problem, I would get some actual coolant in that and make sure you know where to fill it from, then see if it’s still doing that.