It's probably a reflection from the filter. This lens seems particularly susceptible to this phenomenon.
I had a UV filter on mine until I experienced too many cases of reflections when there are light sources in the frame. I ended up taking the filter off and shooting bare. I'll use the filter again if I ever shoot under extreme conditions with the lens (sand, dust, drizzle, etc.).
The reflections are easy to see through the live-view, just point your lens with filter at a light source. When you see a reflection, take the filter off to compare. I'll bet the reflection goes away when you do that.