Two thoughts:

1. Try checking for the leak as soon as the engine is warmed up. Then it won't be too hot down there at the inlet. If it gets too hot the coolant will evaporate. Had a somewhat similar problem with my 240 years ago. It would leak through the waterpump seal. Could only see it when the engine first warmed up. After that the cylinder head's heat would evaporate the coolant. Took me a long time to find that one!

2. Have you checked for leaks in other locations? One location for other Volvo engines to leak coolant is the small hose going into the coolant reservoir bottle. Also check for micro-cracks on the bottom of the bottle itself. Finally, thoroughly check the large coolant hose to the block. That one gets a lot of heat!