I have a 2015 Ford Focus with 44k miles with the 1.0 engine. I noticed that the overfill tank was below the min line so I topped it off. Couple days later the level was back below the min line. Do not see any leaks so assume it is burning it. Are these engines known for this?
Are you checking with the engine at the same temperature? The amount of coolant in the tank will change as coolant expands and contracts.
While I don't know if those engines are prone to head gasket problems I would suggest pressure-testing the cooling system. If you're lucky it just may be a loose clamp or pinhole leak in a hose. Frequently those are otherwise difficult to spot.
If that doesn't turn up anything you can try one of those coolant testing kits that that Scotty has talked about to check for exhaust gases getting into the cooling system. Another thing to take a look at is your oil and oil level for signs of contamination or oil level increasing.
And, you can change the radiator cap with a new OEM. A cap that doesn't pressurize the radiator would leak fluid.
There's also a block drain on most engines for coolant. (This is separate from the radiator's own drain plug). Maybe it's a bit corroded and leaking.
Check the upper and lower radiator hoses as well.
Scotty has a video on using UV dye to track possible coolant leaks: https://www.youtube.com/watch?v=rDuGeQnMOzk
If you can't see or smell and external coolant leak. I'd check the oil dipstick...make sure it's doesn't look like a milkshake. If that looks normal, then further checking/testing to find where it's going.
Also make sure your overfill tank cap isn't damaged
