Instructor is utlizing hidden topics in their course. When an activity is set to show in a hidden topic/section, the Activities block picks it up as available and shows the link to students.
Steps to reproduce:
1. Create a course and enroll one student
2. Create an assignment (for example)
3. Hide the section that contains the assignment
4. Set the assignment to Show (either in the options menu for the item, or in Edit Settings > Common Module settings)
5. Add the Activities block to the course
6. Login as the student
Expected: Example assignment should not be available to the student until the instructor opens the section
Observed: The example assignment is available to students despite the section being hidden
*By default, when hiding a section, it hides its activities too. These are manually set back to 'show' in the hidden sections, though.