i had a simular problem and couldn't find a single leak but my coolant went down all the time....
turned out one of the hoses had a very small hole that didn't leak when the engine was cold, but when it got warmer and the pressure would build up it would start to leak. They're very hard to find leaks and if i hadn't stumbled upon it while doing something else i probably would have never found it.. So basicly stay positive, doesn't have to be your headgasket..

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