Extra collapsed fieldset on node form
Your first option is to simply inspect the html of the form, it should have identifying markup in the HTML which you can then use to grep all your modules for.

Your second option is to implement hook_form_alter and output $form to see which form is generating it and whether there are any identifying markers there. You can use kpr() to output the results of $form using the Devel module to get nicer looking output. You can also use hook_form_alter to remove the fieldset if you want.