episerver index content

I am trying to restrict data to be indexed until it is linked to any content. I have followed the following link http://www.herlitz.nu/2017/01/24/index-only-linked-documents-using-episerver-find/.

It works fine until I have the Block which has a content area which stores GenericMedia type items. I have added a debug point in here

ContentIndexer.Instance.Conventions.ForInstancesOf<IContentMedia>().ShouldIndex(x =>
{
    return false;
});	

When I have a local asset for a block then it hits that break point and I can have my logic of checking if there are any refrences for that item. 

BUT when I have a global asset it doesn't hit that break point. I can't have custom logic to check if I should index that item or not. Any help?