That probably is the most likely explanation. When unused, the belts do lie nicely on the seat, but they are still a bit loose, so I guess there is a possibility they could slide over and hit the hard plastic. My previous car used to do that all the time. I did a word search on their .pdf manual for "belts" but it did not mention the notch. Kia adds a lot of little, nice touches. I wonder how many other "easter eggs" there are?