I've recreated the problem. It returns no results if you do not have the 'VIEW SERVER STATE' permission. Looked it up in the sys.dm_db_missing_index_* entries in BOL. server-level sysadmin has that right bundled in. Not sure about which of the others. dbo (database owner) does not (it does give 'VIEW DATABASE STATE' though, which is insufficient for sys.dm_db_missing_index_*). I'll update the documentation.