Vauxhall omega 2.5 V6 losing coolant?

I get the "coolant level low" warning coming up on the vehicle computer screen, the temperature gauge is normal. Header tank empty but no sign of water under car. When I refill the header tank warning goes out, but have to top up about a pint after every journey. No sign of coolant in oil which appears uncontaminated and remains at the same level. Engine starting and running OK?