Figured it out.  It was the blue led button you press that tells you which machine you are working on by flashing a blue led on the back of the rack.
I just received a Precision 7920 Rack workstation from the Dell Business Outlet today.  Everything works well and passes all tests with the Windows Dell Precision Diagnostics software and Dell Support Assistant software.  Annoying issue is the Status lights on the left front and rear both flash blue (about one second on, one second off).  I can't find anything in the documentation about a flashing blue, only steady blue, flashing amber, etc.  What does this mean?  
Precision 7920 Rack Blue Status Light Flashing