I had my camera sensor cleaned by Canon about 4 weeks ago and on getting it back I took the body cap off and connected my 24-70mm lens and it’s never been off. Last week marks started forming on the sensor and then a few days ago this huge chunk of crap was on the sensor.
You can see the effect of it on the stitched image I took just to see how it would turn out.
I took it to a local camera tech guy (Tim) and he pulled out this small chunk of black plastic off the sensor and then removed some other small fragments. We came to the conclusion that somehow either when Canon put the body cap on or when I took it off the cameras lens chassis has shaved off some plastic from the body cap and it has fallen into the sensor well. By luck the small piece of plastic on the sensor was the best place for it. He said it could have easily made it’s way else where and cost me $500 + in the end it cost me nothing to have cleaned. Phew. The piece of plastic was about 3mm x 1 mm in size.