[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.