Samples from a recent shoot I tried using the FS100 on. We has to shut it down and get a different camera because of these sensor reflections.
It's the image reflecting off of the sensor and onto an element of the lens.
All of these are with the stock lens and Nikon still lenses. With and without filters (mixed throughout).
CORRECTION: the first shot is with an old Nikon, not with the stock zoom as noted (notice the reflection is upside down). The rest of the shots are with the stock zoom lens (notice the reflection is right side up).
Also note that with the stock zoom the reflection is not stabilized, suggesting that the sensor is reflecting from a forward interior lens element.