If a record is catalogued but does not successfully get indexed by Elasticsearch, it is called 'missing'. A summary of this can be seen on the Koha About page, under the System information tab. A user may see something like this: Records are not indexed in Elasticsearch Warning 27 record(s) missing on a total of 1807413 in index koha_kohadev_biblios. However we can't see from this view which record(s) are missing from the indices. It would be great to have a list of the IDs that are in the Koha database, but not in the ES indices. Probably need this to be a cronjob as with large catalogues, a request like that would crash Koha. So would also want to report the time the script was run last to the page as well.
Forgot about the /misc/maintenance/compare_es_to_db.pl script, which does this already!