If the monovalve has been closed all this time (unlikely), you could simply have air in the heater core.

That engine is notorious for not filling completely unless you work at it, so I'm not surprised you are low after running the heater some. Did you get gurgling sounds in the dash? If so, definitely air in the heater core working out.

I'd also replace the pressure cap if not done recently, they go bad and allow coolant to burp out into the overflow tank under the fender.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!