I have done this on many different vehicles. A good clue is the system being full when you've only added, say, half what it should hold. Often the cause is the thermostat being airbound and refusing to open because it isn't immersed in coolant, just slowly heating by conduction, or radiation when the thermostat uses the rubber perimeter seal common on many makes. Bmw's often have an air bleed at the top of the radiator and Saabs have a bleed nipple on the thermostat housing. A good shop manual will have this info.
Several years ago, I was mystified by "The Puzzler" on Car Talk. I know their true profession is being comedians but it can be quality amusement. I missed the following week when the answer was revealed but thought about it several times. The answer finally came to me several weeks ago. The scenario was an old-timer replacing a thermostat. Another mechanic saw him open a bottle of aspirin and asked him what was hurting. He replied that he needed it for the job. Why?
He put a couple in the thermostat to jamb it open. This lets air past preventing airlock and they dissolve the moment coolant touches them. I will try this next time I replace the coolant on my Volvo, the most recent airlock victim.