I had the same issue twice. Fisrt time it was the water pump and the valley pan gasket. The second time it was the expansion tank. Both times the coolant would leak out very slowly and I wouldn't have to add any for a significant number of miles. The expansion tank was more obvious because the leaking coolant left a white color residue on the hood liner and some other plastic engine components. I would have the cooling system pressure tested if you can't determine the cause of the issue easily on your own. Beter safe than sorry. You don't want to let the engine overheat if you aren't paying attention.

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Profeshenal spellar