When you say top up..... You do leave at least a pint of room for expansion, correct?? If you top it to the top, it will blow a pint or so out as it gets hot and expands.
Or.. you could have a leak in the heat exchanger and the coolant is going out the exhaust
. We (who do it for a living) carry a pressure checker with us and would put it in place of radiator cap and see if you are loosing pressure. If not, no leak.
If the coolant is not under the engine, not under the hot water heater, not in the bilge, not mixed with engine oil
, its going out the exhaust
. Bad heat exchanger. Or... some of them have a gasket
between the coolant and salt
and maybe leaking there.