I flushed the coolant in my parents 850 a little over a week ago. I topped it off with Volvo coolant and distilled water, and the car was good to go. Over the next few days I made sure to top it off, and eventually the coolant level stabilized right under max. The car was driven some 300 miles, and the coolant level remained unchanged. This morning (after my parents took a 400 mile trip yesterday)I was checking the oil level, and noticed the coolant level was in-between min and max, a little closer to min. I checked the level again this evening, after the car had been driven some 20 miles, and it was still right in-between min and max. I checked the entire cooling system, and there are absolutely no leaks. I was wondering if perhaps I was just checking the level when the circulation process had the expansion tank level a little "low." I should state that the car is running at normal operating temperature, but I just can't figure out where the rest of the coolant has gone. Thanks for the help :)

I should also add that one year ago the radiator, as well as the expansion tank and cap, as well as the hoses, and the t-stat, were replaced.