Did someone sample my blood? Left a carving of a triangle?

I was practicing on a birch tree, punching it for fun. I had scabs that opened, so i left a lot of blood on the tree. The tree was by the mailbox, so id see it often. I went back the next day, my blood was completely gone. On the tree, a carving of an equilateral triangle.

What do you think this means? It still puzzles me to this day.

I lived in an apt complex.