Can't locate back-end code for shortcode in content

I’m working on an existing WordPress site and the content editor on a Page shows the following:

[HTML1][HTML2]

Each of these loads content on to the page since if I remove one or both, some of the content disappears.

Problem is, I can’t find the code that this shortcode is related to.

I’ve used a snippet plugin before, but this site doesn’t use one. I checked the back-end searching on do_shortcode, but I can’t find the code that relates to this shortcode.

The code that displays the content from [HTML1] looks like:
<?php $welcome = get_post_meta($post->ID, "welcome", true); ?>

But there are no custom fields on the dashboard for this page, and Advanced Custom Fields isn’t being implemented either.

Any advice on where I might search next to see where this code is coming from?