Canon News has discovered an interesting patent that identifies external accessory based cooling for a 1 series camera body. While this camera shown in the illustrations is clearly a 1 series camera, it does not appear to be the 1DX Mark III body. It’s highly possible that this was one of the early attempts to manage heat in the upcoming 1DX Mark III that was later abandoned.
The goal of this patent is to find a way to cool down the camera without increasing the size of the camera, as explained from the patent application;
Conventionally, even in a digital camera, heat generation in an image sensor and an image processing circuit has become a problem due to an increase in pixels of the image sensor and a high frame rate of a moving image. In addition, the camera itself is required to be miniaturized, and there are few parts that release heat. For this reason, there is a limit to heat countermeasures by using heat transfer members that have been conventionally used to diffuse heat to the respective members and making them uniform.
For this reason, forced air cooling using a fan or the like has been proposed, but if it is built in, the image pickup apparatus itself may become large.
Canon explains in the patent application that with a combination of creative engineering around the viewfinder, and a fan-based accessory cap for the optical viewfinder, they can manage to provide cooling to the camera body itself.
Using an accessory cap for the viewfinder makes sense because when shooting video (which is when much of the heat would be generated), you wouldn’t be using the optical viewfinder anyways.
Maybe this is actually a thing, and to cool down a completely sealed camera like the 1 series, Canon has to get creative.