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.

This sounds normal to me. I have noticed that the coolant level in the radiator typically drops when you warm up the car and the themostat opens. I do not believe that that there is a bleeder valve on the Volvo. To refill mine, I park on an incline with radiator facing uphill. I fill the radiator to the brim. I then start the engine and run until it reaches normal temperature. Open radiator again and top off again. Then I fill the coolant reservoir to appropriate level.
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1998 Volvo S70

I was thinking that it could be a air pocket that caused this, and had someone on another Volvo forum second this idea. I am going to top off the coolant tomorrow morning, and keep an eye on it for the next couple of days. Have a most splendid evening one and all :)

It appears that I am to blame for this, which should really be no surprise. When I was taking out excess coolant last week, with a turkey baster used only for extracting coolant, I must have caused a few air bubbles. This morning I topped off the coolant, and true to form I put in a little too much. Tomorrow morning I will bring it back down, while using the baster properly this time, and I should be good to go. One quick question: where should the coolant level be (in the expansion tank) when cold? Thanks!