In a Moodle course site create a recurring scheduled chat.

In "Edit course settings," set "Course visibility" to "Hide."

Return to your dashboard page and look at the "Upcoming events" and "Timeline" blocks.  The chat session(s) will appear there.

This seems to happen in the teacher role, but not in the student role, but it seems to me that it shouldn't be happening at all, that NO activities in hidden sites should be visible in the Timeline/Upcoming Events blocks.  We're seeing this happening in Moodle 3.10; it was not an issue with Moodle 3.8.