Having sorted out my 3 Astras, I return to the coolant loss in the Vectra.
I have tried 3 different caps, no cure.
No steam from the exhaust, or sign of water in the oil (not milky). Oil level not rising, not falling much either. Carpets do not feel wet, though I have not peeled them back yet.
I removed the front bumper, but all that revealed was the aircon condenser. Ran the engine for an hour with the bumper off and new cardboard under the engine - no sign of leaks and no wet patches on the cardboard. Coolant level remained the same for the hour, but fell on cooling. After lunch topped up coolant, went for 12 mile drive, returned home, no apparent coolant loss.
Haynes says the electronic throttle makes compression tests difficult. I considered draining the oil and looking for coolant in it.
No sign of coolant near the water pump.