As long as your indexer supports paging and it hasn't been too long Radarr will be able to process the releases it would have missed and avoid you needing to perform a search for the missed movies.
If the movie hasn't been released yet, you do not need to initiate a search.
Instead, it fairly frequently queries your indexers and trackers for all the newly posted movies, then compares that with its list of movies that are missing or need to be upgraded.
The RSS feed is comprised of all releases recently available, including releases for requested media you do not follow, if you look at debug logs you will see these releases being processed, which is completely normal. The RSS feed is the latest releases from a site, typically between 50 and 100 releases, though some sites provide more and some less.