I have a SP List with a few metadata, no special functionality involved. The New/Edit/Display forms had been created using Nintex Forms 2013 (recreated; originally it was created with InfoPath) and on the Display form (only the diplayForm) there is a "content panel" (content part) appearing - however it is not a part of the form and I can't even figure out where is it coming from. I did not create the form originally, so I do not have more info on what was involved
Any idea where it might come from, why I can't see it in the From settings and/or how to get rid of it (when it's not in a Form settings)?
Unfortunately I do not have an access through SP designer/Visual Studio
Are you using a form viewer on your aspx page?
Perhaps there is another web part on that page that you could delete.
Can you open it with sharepoint designer?