@dullahir - to search for books that do not have a series associated with them, just type this in the search bar for calibre:
The reason it isn't in the QC plugin is because of course only a fraction of books people normally keep in their libraries are part of a series, so it is a fairly "unusual" search. It would be trivial to add it if others think it would be useful.
On the other hand there is on the Check metadata menu the ability to search for series gaps (where you have book 1 and 3 for instance, but appear to be missing a book 2).