I don’t see a leak but the coolant will be reduced to half in the reservoir in 3-5 days. What could be the cause.? What should I have checked before considering changing the engine. Thank you

There's a few ways coolant can be lost while driving that won't leave puddles on the driveway. 1. Leaking heater core, lost coolant can then drain to ground via drain tube. 2. Blown head gasket, coolant can be going out exhaust, or draining to ground via overflow due to excess pressure in cooling system. 3. Bad radiator cap, doesn't maintain pressure in cooling system and some coolant is lost via overflow drain. Things to check: run a cooling system pressure check. It will find leaks (this requires special apparatus, you can buy or rent, or just take it to a shop.) Compression test, this will check head gasket. Block tester (this will test coolant for presence of combustion gasses, another sign of a blown head gasket). Block tester can also be bought or rented at any auto parts store. Check the oil for emulsions (dipstick or fill cap). Check PCV system also for signs of emulsified oil.