[rjSupra]

Just to chime in something here, but I am certainly no expert, but I've read here that systems should not lose coolant Period. If the levels need to be replaced like in a year or two at all, then you have either a leak or a BHG aka the blown head gasket. I don't know your car, but I doubt it's a BHG. That's special to us Supra owners due to a factory error. But I'd bet you have a coolant leak somewhere. If there is no leak at all and you are sure of it, check your oil for milkiness in it, not pure oil, and look for smoke in your exhaust - the symptoms of BHG.
I'd run it and take it home and put some white cloth or cardboard underneath it overnight and see if there are any stains or fluid on it at all the next day. You might have a small lose fitting on a coolant hose somewhere, that only starts leaking and spraying when the motor is warm, the thermo is open, and the temp and pressure is way up to force it out.
That's essentially what happened to me, at least I think so. I'd lose tons of coolant after full temp and running the motor at over 2000 rpm on highway, then the coolant would let loose everywhere and I'd overheat. I replaced the water pump, the thermostat (was coming out around that area) but did not fix it. I have a small very difficult to see small coolant hose that I am replacing tomorrow and another one that I know isn't leaking to be safe, and the gasket to the water outlet housing (where the thermo bolts into which bolts into the head). I have a full long saga posted here if you want to read more.
Good luck