It appears that stealth activities is visible to users/students when going to activity index page in a course.

Steps to replicate.

1. Create a course.
2. Turn edit mode to On.
3. On section 2 add a quiz.
4. On the course contents page, click "edit" and "hide" the quiz. The quiz will now be displayed with message "hidden from students"
5. Add another quiz and save and return to course.
6. On the course page, click the edit button for section 2  and click hide topic.
7. RESULT: All quizzes includin the section would have a message "Hidden from students"
8. 
9. No click edit button for the first quiz and click "Make available". The message should now be displayed for the first quiz "Available but not shown on course page".
10. Log in as student or switch role to student.
11. Visit and list the quizzes for that course using the format URL/mod/quiz/index.php?id=[courseid]. 
12. RESULT. Users/students can see and access the first quiz.
13. 
14. Go to the course page, and it is not visible (because it is stealth activity.

However, per description of stealth setting it says

Allow stealth activities allowstealth

Default: No 

If enabled, activities can be made available but not shown in visible sections of the course page. If so, links to stealth activities must be provided from elsewhere, such as from a page resource. Stealth activities are still listed in the gradebook and other reports.

Perhaps the stealth activity should be hidden as well from the mod/[activity]/index.php page as well?