>But when he uses 1/2 shutter to >focus there is a red flash visible on the edges of the >viewfinder image.
I've noticed the red flash occasionally on my D600. I'm virtually certain that what we're seeing is simply the red of the auto-focus assist light which comes on to aid auto-focus in low light conditions. I believe the auto-focus assist can be turned off, so if the red flash disappears when it is (which I haven't done yet, although I'll try it the next time I'm focusing in low light), there's the culprit for sure. Naturally, I'd be interested in anyone else's thoughts here.