LensRentals has posted an article about sensor issue on some of new Canon EOS Rebel T6i & Rebel T6s DSLR cameras. After a closer examination with a 10X microscope show the spots are inside of sensor, within the stack and under the top layer of glass.
According to LensRentals:
The bottom line is that 4 of the Canon T6s and 2 of the T6i cameras we received had to be sent back because of a defect in the sensor stack (the layers of filter glass over the sensor). This is out about 10 copies of each; the others were absolutely perfect.
The affected cameras all had a dramatic pattern that at first we thought was oil or dust on top of the sensor glass.