There is an option to notify students when there are feedbacks posted in an open-ended question on a graded test by instructor and also an easy-way for instructor to find those comments.
As an instructor, go to test reports and click on the 'Grade' button to open the questions review page.
In the questions review page, click on "Questions feedback" option to filter only open-ended questions that has a feedback written by the instructor.
Furthermore, instructor can setup notification feature to notify students when instructor adds question feedback for open-ended questions.
To setup student notification e-mail template, click on "Test Notification Settings" link in the test options gear icon.
Now, click on the "Student E-mail Notifications" second tab and enable the template high-lighted below:
The e-mail template can be customized with variables and custom text.
Similarly, you may also setup other email notifications and trigger an e-mail based on an event.