Getting ACF field name to appear in search

Relevanssi Premium stores the custom field name in the index, so you can see that for post X, the word Y appears in the custom field Z.

The free version of Relevanssi does not store that information. You need to query the wp_postmeta database table to see where the term appears. That’s straightforward, but you’ll have to see how slow it is – the postmeta table is often quite slow, so especially if you have lots of pages in the results, some performance optimization may be necessary.