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Well, that's some good news. The bad news is that you really need to find out where that coolant went. Could be anything from a bad head gasket to a leaking hose, radiator, overflow tank, etc. Is there any noticeable signs of coolant anywhere? Any white smoke lately with a sweet smell to it? Easiest is to do a visual check. I would start with checking the hoses, heater core lines, bypass hose, t-stat gasket, radiator tanks for seepage, overflow tank for cracks or a bad hose, etc. If you can't find any visible external leaks, then it's time to start looking for internal leaks.
Has the cooling system been flushed recently? An improper fill could also pose some of the same symptoms if there was air trapped in the system.
Backflush that crap out of your radiator unless you really want to kill your engine.
Take a GOOD look at a head gasket if you want to see why.
And replace that radiator.
Why backflush and what's wrong w/ my head gasket?
Stop Leak will plug small leaks in radiators and also small water passages in your engine. Thus stopping proper coolant flow.
Not your head gasket, at any head gasket. You will see the small holes where coolant flows from the block to the head. Stop Leak will plug or restrict these passages. Less coolant flow = hotter head = CRACKed or WARPed head.
Flush it and replace that bad radiator.
My advice would be to replace the radiator now. They're not that expensive and your cooling system will thank you.