He even drew it in that hole they threw him into on the senior camping trip. I wish they would have explained that one. Or maybe it was just for us so we could link together everything that Clay was doing. It was his signature. But I still think it's from his comics too.
If you look at it, it kinda looks like an A and Z which could stand for Alex and Zach because they were the main 2 people who injured and killed Bryce. So then the 2 dots would also represent 2 killers. Thats just what i think though so i don't know if it is true or not :)