Is it sticking out above the surface of the roof, or an indention? From the looks of it, it's proud of the roof, so probably there is something underneath. It appears like it could possibly be something that was spilled on the roof (glue?) before the roof was put down. I'd guess that if it was glue, it was allowed to dry then the roof was installed. Probably won't be an issue unless something catches there & rips the roof material. But, I'm no expert, just thinking out loud, for what it's worth.