how to locate a line of code appearing on the site?

Hi All!
I have spent about 2 hours trying to find the answer for my problem in forums, but I guess I am missing the good keywords for a start, since I dont know how to correctly describe it to search.
On my localhost version I get this line

‘header-menu’, ‘container_class’ => ‘header_menu_class’ ) ); ?>

appearing on the bottom of every content in every page. I assume I have at some point wrote in one of the files something wrong, but how can I locate the source of the error to fix it?

All I can find is tips on how to ‘hide php notices’, etc. But I would like to be able to locate what is generating this line of code to appear, fix it.

I have already used a php checker online and it did not point out any error like this.
I have also open all files on Notepad ++ and searched in All Files for that line of code, but it is also not there.
I have been doing some changes offline and I have not noticed since when this line appears, so I cannot know anymore what change in header, or function, or style.php cause this. Also when I check those files, nothing spot out as wrong.

I tried to inspect on the browser, but also it does not point out.
Is this a so-called PHP notice? How to trace it and fix?? I am hopeless…

Thanks in advance!