When a student is enrolled in multiple courses, it is possible when an Instructor has hidden a Tool link that would take the student to a particular tool in one course (i.e. My Grades), that if a student uses Course-to-Course Navigation and enters a different course where the link has not been hidden, clicks on the link which accesses the Tool and then uses Course-to-Course Navigation to return to the course with the hidden link, the student would bypass the hide feature and go directly into the Tool, giving unintended access to the Tool.
Note: This also applies to Course Menu buttons which have been hidden and link to a Tool.
The only way to prevent this from happening is for the instructor in the course with the hidden link (or button) to actual disable the Tool by going to Customization>>Tool Availability, instead of just hiding the link (or button).