Water antifreeze, goes through engine cycle, if leak was daily, then you would see leak, and smell odour, the first idea may be heater core gradual leak and going into cab, and leaving film on windshield, then the worst idea, being a leaking head gasket, but you would have loss of compression and black smoke in exhaust ,
Another idea is that it is evaporating through rad cap as steam, but really, you would see steam, and there is an over flow hose from reservoir, if fluid over expands and force out through hose, but that means engine is really hot, or resovoir is overfilled when hot,
Other glitch would be the wrong antifreeze(pink) was used which can cause heat and damage to wrong vehicle,
Other things would be the thermostat open or block or radiator blocked, or water pump corroded and not push fluid right through causing top of engine to overheat, that causing possible pressure leak in gasket,
You can try running then idling, check engine area, check exhaust, check defrost and odor,
Still leaking and can not find source, take to shop for check,
Probably getting vaporized in combustion (head gasket or cracked head). Have a look all around the engine see if you see any colorful leaks – crawl under and look up he sides of the engine too – dried anti-freeze leave tracks if it leaking outside the engine but not making it to the ground.
2 possibilities –
a – water only leaks when engine is hot and immediately evaporates due to hitting hot parts. Look for tell tale runs of residue
b.- blown head gasket meaning coolant exits via the exhaust. may be slow enough not to be too noticeable except in very cold weather or maybe not at all.
Possibly coolant is getting into the combustion chambers and out the exhaust. It could be a blown head gasket.
Check your oil. If it's a grey foamy mess, that's where the water is going. You need a new head gasket.
Do cooling system pressure test.