Some Twitter users on iOS or Android can now see if any of their Space followers are listening, right at the top of their feed.
Twitter apps were already showing Spaces in the top bar (you could also see Fleets there before Twitter shut them down), but these were Spaces created only by the people you were subscribed to. But now you'll not only see the Spaces they've created, but also the ones they've just stopped by to listen to.
If you don't want your subscribers to know which Spaces you follow, you can disable this feature by going to Settings and Privacy, then Privacy and Security, and then disabling the "Let subscribers see which Spaces you follow" option. However, disabling this option will not hide your presence in Spaces to its members, this is something to keep in mind. It is not possible to visit anonymously.