There are a few reasons why content may not have a menu entry. Perhaps the content is in an unpublished state, whereby a menu setting will be automatically disabled. Or, perhaps the content may be accessed via another method such as a view.
In any case, there is a way to locate content that does not have a menu entry.
To locate content that does not have a menu entry, do the following:
Go to Admin menu > Content management > Content.
Filter tools are available at the top of the page and can be used to help thin out the list of available content.