This is my understanding, hopefully it is correct.
Before the most recent update, the SBIG driver caused the behavior you described, that is, it acted like the guide sensor was literally "internal" whether it was actually behind the shutter or in front of the filters. On the SBIG forum at the time, they stated that this was necessary to keep the driver compatible with older style cameras/guiders.
Mine behaves just like @pfile mentioned. Guider shutter opens and closes on every exposure until an image starts.
I took the recent SBIG release description to mean that this has finally been decoupled so that the driver now knows that it does not need to open any shutter to capture a guide frame. I haven't had a chance to update the driver and try it, but I hope to over the weekend.
Hope this helps,
Craig