Does update_comment_meta hook exists?

Well, it’s a little bit complicated 😉

If you will search update_comment_meta hook, you won’t find it.

But it exists 🙂

There is update_comment_meta function and it calls update_metadata function with first param set to comment.

This update_metadata function does something like this (meta.php line 149):

do_action( "update_{$meta_type}_meta", $meta_id, $object_id, $meta_key, $_meta_value );

where $meta_type is the first param of this function.

So it will be update_comment_meta when this function is called by update_comment_meta function (because $meta_type is then equal to 'comment').