Since 2008, Apple has been installing Apple Server Diagnostics 3×106 on Xserves and Mac mini servers on a hidden partition of the boot drive, accessible if you hold down the N key during the boot sequence. This is similar to the Apple Hardware Test that is installed on a hidden partition of the boot drive on Mac workstations.  With both, if you reformat the boot drive, these hidden partitions are lost.  Cursorily it would appear Apple is offering a download for the Apple Server Diagnostics 3×106 if you’ve lost them, available at: http://support.apple.com/kb/DL948