1995 Honda Civic Loss of coolant

For two years now, this car has been slowly losing coolant. We just check the coolant level about 1-2 times a week and replace as needed. No wet spots anywhere, around the engine or the ground. No evidence of steam loss after shutting off the engine. No heater core leaks. Recently replaced radiator and cap, thermostat, hoses, and coolant switch. Coolant fans work. Water pump was replaced 20K miles ago. Compression checks out OK. What's left?