[OhArgh]

There is something hidden by a black square at the far end of the vehicle.

[nullc]

Nothing is 'hidden'— the image is a stitched panorama from many smaller images, and they didn't (yet?) transfer some of the images which only cover the rover, presumably because looking at the rover is scientifically uninteresting. The black areas are just the boundaries of the missing images.

[smackfu]

It seems to just be a glitch in this copy of the pano. This other one is fine: http://www.360pano.eu/show/?id=731

[seanlinmt]

wouldn't that black spot just be the mount that's holding the camera?

[Synthetase]

That's definitely the MMRTG or its nuclear battery. My guess would be the Department of Energy doesn't want too much about it floating around.

[biesnecker]

Though it's not like photos of the MMRTG don't exist (e.g., http://spaceflightnow.com/atlas/av028/111117mmrtg/)

[nollidge]

There are already lots of pictures of it. Quite sure the DoE doesn't care at all.

[nikic]

What are the reasons for hiding it?

[OhArgh]

It's odd. I just looked at it again and now the square is gone. Maybe as suggested it was a rendering issue and it was just a coincidence that its was over the battery.