This is a puzzle! I can't give you an answer, but it is important to know that overheating in this car with the aluminium block is extremely hazardous. This is why Volvo also installed a coolant level warning light....it wasn't to be glitzy. So if it was occur again, respond immediately with what you did previously. Do not wait. That said, the cooling sysyem is one of the most simple in design and operation in the car.

If there is any loss of coolant, put the system under a coolant pressure test, both cold and hot engine. If the coolant hasn't been changed in 30K (miles), change it with Volvo coolant and a new thermostat. Hoses and pump would be unlikely culprits at this age. It also may be that you would want to downshift to the '3' position while driving up those steep hills. This will keep the engine rev'd, fluids flowing and the tranny not working so hard.