I always load my flight plans in the INS only, so even if there was a hidden GPS unit, it wouldn't know where to fly to.

But, we can easily check if there is such a thing: Open the panel.cfg and look for any "gps" unit there (should be a gauge then). You won't find any, so I think you are seeing something different here.

Maybe the Doppler gauge uses some kind of function here while the INS is completely independent by design.