I had the same issue. My stinger never overheated but only reason why I decided to check cause I had a smell coming from the rear. However it did not smell like coolant at all.
Anyway I decided to pop up the hood and I looked and boom this is what I saw. Went to KIA dealer and bought coolant. I filled it to little above the L line. Drove it for a day and it dropped again. But for the life of me I could not find a leak anywhere and car runs fine. I wonder if I should take off the radiator cap and top it off? I tried taking it off initially but I could not get it off so I left it alone.