I haven't been checking mine but, after reading your post, I think I will in future.
The manual states:
"The coolant level should be filled between the F and the L marks on the side of the coolant reservoir when the engine is cool.
If the coolant level is low, add enough distilled (deionized) water to bring the level to the F mark, but do not overfill. If frequent additions are required, see an authorized HYUNDAI dealer for a cooling system inspection."
The small amount of water you add to the coolant to bring it up to F won't make a big difference in the antifreeze protection. Maybe measure and keep a record of the amount and date you added to the reservoir.
If you want to, add a 50/50% mix of distilled water and antifreeze (ethylene-glycol with phosphate based coolant):
The manual states:
"The coolant level should be filled between the F and the L marks on the side of the coolant reservoir when the engine is cool.
If the coolant level is low, add enough distilled (deionized) water to bring the level to the F mark, but do not overfill. If frequent additions are required, see an authorized HYUNDAI dealer for a cooling system inspection."
The small amount of water you add to the coolant to bring it up to F won't make a big difference in the antifreeze protection. Maybe measure and keep a record of the amount and date you added to the reservoir.
If you want to, add a 50/50% mix of distilled water and antifreeze (ethylene-glycol with phosphate based coolant):