I feel some are overthinking this, thinking that if it's not in your sitemap then it won't get indexed, that really isn't the way Google and other search engines work, a sitemap is basically to kick start indexing. The goal for any site is to get the content indexed, a sitemap kick starts this and provides info for search engines to get started indexing, as they do, they will naturally index the whole site based on links, etc on your site as normal.
I wouldn't think of a sitemap as a must have for every single item, think of it more as a way to push start the indexing process and let the search engines do their thing by indexing the complete site as they are crawling. If a topic is being crawled, search engines will certainly pick up the replies to this topic and index them too. 🙂