データベースがオフラインの時にdrop database[DATABASE_NAME]をすると物理ファイルは削除されないらしい。
*Dropping a database deletes the database from an instance of SQL Server and deletes the physical disk files used by the database. If the database or any one of its files is offline when it is dropped, the disk files are not deleted. These files can be deleted manually by using Windows Explorer. To remove a database from the current server without deleting the files from the file system, use sp_detach_db.*