Surely 'breaking the fourth wall' is really a deliberate act on the part of the writers and the characters concerned, usually as a device to bring the audience into their confidence.
I contend that the apparent breaking of the fourth wall in CS is merely, as others have suggested, a straightforward staring into space expression rather than a genuine look to camera.
Trouble is, with HD TV, there's no hiding place for this kind of inadvertent eyes roaming .. if that's indeed what it was. We the viewer don't miss anything these days, not the tiniest skin blemish thickly covered in make up, nor poorly applied mascara clogging up the lashes, etc.. On the male side, poor shaving can clearly be seen - and I don't mean so called designer stubble (that's just sheer laziness). Injury make up needs to be pretty good as well these days to be truly convincing.