Ok , it could be the glass ...
Just depends on when it shows up .. ( pictures )
I have personally had it myself , and it was the result of cleaning the sensor and leaving streaks ...They usually only show up under certain lighting conditions ..
Macro has a way of bringing out all the tiny imperfections normally not seen ( sensor ) .
I usually take a picture of the sky ( blue ) and then in post production use Auto color correction , and it shows all the dust on the sensor . Quite an eye opener at times ( like just having purchased a used camera )
---------- Post added 09-18-15 at 01:16 AM ----------
Blue sky shots usually show up dust that are going to be a problem ...
Auto color correction shows up most dust particles sitting on the sensor , and it can be a real PITA to get the sensor truly clean ..
Because its Macro , small aperture , and flash that show these hidden gems ...
Those into portraits and landscapes , dont usually suffer from dust as much ...