I've also had good luck hunting for non-obvious leaks with a coolant UV dye kit (UV Glasses, UV flashlight & UV dye) found on Amazon. You may have to buy the dye separately. Make sure you get the dye for cooling systems, not oil.
Put 1/2 bottle (1/2 ounce) in expansion tank and run it for a day or two ..
Then at night put on glasses and use flashlight to find UV source.
I just found my leak at the heater control valve...
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