In the case of using depth-mapped shadows, what you are seeing is called a "light leak" or incorrect shadow offset.  Depth-mapped shadows can always suffer from this kind of flaw depending on the arrangement of objects in the scene.  You can adjust min shadow bias and try to minimize the effect, but can't ever prevent it from happening.  It is one of the fundamental flaws of using depth-mapped shadows.  There are other flaws with depth-mapped shadows aside from that, notably various kinds of artifacts and fundamentally wrong blurred shadows.

http://www.peachpit.com/articles/article.aspx?p=486505&seqNum=5

They're okay for low quality work, or cases where speed is a high priority, but imo if you want high quality lighting you should get away from using depth-mapped shadows entirely.  The flaws I'm talking about are not particular weaknesses of Poser, they are problems in any rendering application that uses depth-mapped shadows.  This is an example scene containing a light setup that imo is quite a bit more realistic than any of the default lighting setups, feel free to play around with it.

http://www.renderosity.com/mod/rrfilelock/download.php?fileid=33527&key=8122

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