If it was in the morning, what you are seeing is a temperature inversion.  The XM is still only as good as the NEXRAD sites it is coming from and NEXRAD is 30 year old technology.  In the morning a temperature inversion will show as anything from small dots, to large areas of heavy precip, just depending on where the inversion is in respect to the antenna.  If you know where the nexrad sites are, that will help you determine if it is and TI.  Always cross check nexrad data with metars, and satellite data.
Luke Alcorn