Read this on FM and I think he is 100% right.
this finding of a purported light leak on the 5D3, is typical of this stage of the life of a new camera model, in a world where too many buyers of such stuff are nerds rather than photographers (I confess being a nerd myself):
someone sets the camera up in a weird way, ie blocking any light from entering
this someone apparently doesn't realise that this means any further "experimenting" will be basically measuring noise.
But doing so in ways much less useful or professional than the manufacturer would
the same person then proceeds to flash a light at the camera. Woooo, apparently a tiny bit of that light gets in!
Should one therefore panic?
Yes if the person regularly (1) shoots in very very very very low light, and (2) regularly flashes strong lights at their camera while shooting in these conditions.
WHO does this? NO ONE. Astrophotographers DO shoot in very very very low light (not THAT low - nowadays any decent DSLR can capture say Orion, hand-held).
But they do NOT flash lights at the camera while shooting.
Camera makers are of course happy with any additional sale made - but frankly one sometimes wishes there were more photographers out there, and less failed testers / nerds. Puh-lease!