Where would a customization for "Load more" button in subpanel be?

I have no clue, but if I was spelunking I would start by digging in custom fields or extensions to existing fields simply because it's a button...

So maybe look in

custom/modules/Opportunities/clients/base/fields?

or in custom/clients/base/fields?

and see if anything sticks out at you...

Good luck!

No custom fields relevant to Opp. in the system. And by the way, I just found out that in fact the load more button end up showing records from a DIFFERENT account. So this guides my thoughts towards permissions/roles. Can something be afoot there?

My next step (which in hind sight should have been the first) would be to check the subpanel layout:
clients/base/layouts/subpanel/subpanel.php

The components suggest that the bottom of the panel is defined in "list-bottom"

$viewdefs['base']['layout']['subpanel']  = array (
    'template' => 'panel',
    'components' => array (
        array (
            'view' => 'panel-top',
        ),
        array (
            'view' => 'subpanel-list',
        ),
        array (
            'view' => 'list-bottom',
        ),
    ),
    'last_state' => array(
        'id' => 'subpanel'
    ),
);

That suggests that there is a list-bottom view and indeed there is one:

/clients/base/views/list-bottom/

and it defines the 'showMoreRecords' event:

    events: {
        'click [data-action="show-more"]': 'showMoreRecords'
    },

so I would check if there is a custom/modules/Opportunities/clients/base/view/list-bottom

that overrides the default showMoreRecords

When we redefine panel-top we do so by using the override_paneltop_view and defining a totally different panel-top view, there could be something in your code that redefines the list-bottom. So I would also check
/custom/modules/Opportunities/clients/base/layouts
and for good measure if you don't find anything there, also check if something really strange was done to /custom/clients/base/layouts  or goodness forbid clients/base/layout (though no one should be touching anything outside custom, you never know...)

My next step (which in hind sight should have been the first) would be to check the subpanel layout:
clients/base/layouts/subpanel/subpanel.php

The components suggest that the bottom of the panel is defined in "list-bottom"

$viewdefs['base']['layout']['subpanel']  = array (
    'template' => 'panel',
    'components' => array (
        array (
            'view' => 'panel-top',
        ),
        array (
            'view' => 'subpanel-list',
        ),
        array (
            'view' => 'list-bottom',
        ),
    ),
    'last_state' => array(
        'id' => 'subpanel'
    ),
);

That suggests that there is a list-bottom view and indeed there is one:

/clients/base/views/list-bottom/

and it defines the 'showMoreRecords' event:

    events: {
        'click [data-action="show-more"]': 'showMoreRecords'
    },

so I would check if there is a custom/modules/Opportunities/clients/base/view/list-bottom

that overrides the default showMoreRecords

When we redefine panel-top we do so by using the override_paneltop_view and defining a totally different panel-top view, there could be something in your code that redefines the list-bottom. So I would also check
/custom/modules/Opportunities/clients/base/layouts
and for good measure if you don't find anything there, also check if something really strange was done to /custom/clients/base/layouts  or goodness forbid clients/base/layout (though no one should be touching anything outside custom, you never know...)

I ran a grep -R -l command in the file system for showMoreRecords and a separate one for data-action="show-more" and both indicate only one file, the base sugar one. I compared with the original and they are the same, it has not been customized. So.. not custom list-bottom, not layout, not custom button, not a field, where the hell is the modification :((

That's crazy!
You said the Show More shows records from another module

Could it be they hacked the relationship somehow?

Maybe check the vardefs for Opportunities and see if there are any clues in the "link" specifications?

I am truly making things up here...

Ah let me clarify. It is not a different module BUT a different RECORD. Aka you load up account with GUID "A". When you click load more it will load Opportunities associated to Account Guid "B" instead of "A"

It sounds like someone created a custom relationship.
See if this gets you somewhere:
https://sugarclub.sugarcrm.com/dev-club/b/dev-blog/posts/creating-subpanels-with-custom-results-in-sugar-7-5