[davidmanze]

I read a post which makes the light source issue cause more plausible. It might be that with certain artificial light sources, where the AF with at least certain Nikons gets inaccurate, IR plays a role. Might it be that the AF sensor focusses on the IR light, which then causes OOF results? The poster was thinking about an IR cut filter, to test this.

This was also a problem with the  Pentax K5, they put a IR in the K5II which cured it. I will be seriously keeping an eye on this low light business as I will with all matters of AF.