Loses over a pint of coolant in a week. No puddles under the car.
There are a number of possibilities, from a bad radiator cap to a bad head gasket. Has the engine been running hotter than usual? Have a shop you trust run a pressure test on the cooling system. Sometimes an engine only loses coolant under certain conditions such as when it’s been running for a while or under a load. A competent mechanic can find the source of coolant loss and let you know your options.