Yes you are right, SQL Server creates these to keep track of non-index statistics on columns, to capture the statistics even though there is no physical index on that. Since these are not real indexes you cannot use DROP INDEX, you must use DROP STATISTICS. But again it is not a good idea to drop these stats as the system created for the maintenance plan or whatever reason. Btw why don’t you normalize your table, do you really need 500+ columns in a table?