When you grade a quiz by selecting Grade from the drop-down by the quiz, you can restrict the set of users displayed by making a selection from the "Restrict to" drop down box and pressing the 'Search' button that resembles a magnifying glass ( ).
If you select the "Users who have not taken an attempt" option, the list of names displayed may include users who are not enrolled in the course.
This is a known issue.
You may ignore the stray names in the list of users who did not take the quiz. However, if you do not want these stray names to be appear, ask for assistance. If you are at UW-Madison, please contact the DoIT Help Desk. Otherwise, contact your campus help desk.
For Site Administrators OnlyA workaround for this problem is for the permission "Take quizzes (as student)" to be turned off for non-student roles.