I am used to adding controls to a Nintex Forms and binding it to the list column to store the value.

However, I just discovered, that I can add a "standard" control, e.g. a textinput control. And it will store the value the user puts in, even though that control is not bound to one of the list columns.

Is this a bug or a feature?

Where is that value stored and can I access that value from outside the list view form?

Any feedback is much appreciated!

Hi,

Unbound controls are stored in a hidden column called "Form Data" in XML format.

Example: 

The hidden column will come up as a reference in Nintex Workflow after the first form is submitted on the list as this is when the column is created.

If you would like to see what is in the hidden column for an item you can create a Multi Lines of text column on your list configured as plain text and then use an update item action in a workflow to update the multi lines of text column with the contents of the form data column.
Config would look like this.

You can extract elements directly out of the Form Data column using a Query XML Action.
 

Hope this helps.