MastoTip of the day:
Did you know that EVERY Mastodon profile has an RSS feed attached to their account?
That means that ALL public posts originating from a Mastodon account can easily be grabbed/seen/scraped by anyone on the internet.
Not to mention there's 20,000+ Fediverse instances and new ones every day that can be doing anything with your public posts without your knowledge.
Do yourself a favor. If you don't want your posts to be publicly available to anyone at any time, make your posts "Followers only". That's the only way to make sure your posts are private.