So, in an open system, where would the coolant purge out? Thru the coolant overflow?

I filled my overflow tank last Thursday, last night it was just below the add line. Now I have been driving it hard, high rpm on highway, county roads, but that just seems like a lot of coolant to lose in under a week. No puddles in driveway, no coolant smell, oil doesn't look milky. I'm at a loss & it's driving me nuts trying to figure out where it's going