Canon EOS-1D X Mark III Advanced User's Manual page 23

Lens
After detaching the lens from the camera, put down the lens with the rear end up and
attach the rear lens cap to avoid scratching the lens surface and electrical contacts
(1).
Smudges on the image sensor
Besides dust entering the camera from outside, in rare cases, lubricant from the camera's
internal parts may adhere to the front of the sensor. If smudges are visible on images, have
the sensor cleaned by a nearest Canon Service Center.
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