When students drop a class (whether by dropping it entirely or by switching sections) they will seem to disappear from the Canvas course site as their enrollment status is automatically changed to "deleted". Upon request, ITS can manually re-enroll a student as "inactive" instead, which will make the student's grades and submissions visible to the instructor while keeping the student's access to the course restricted.
Once brought back into the course "inactive" the student's records can be reviewed through the process shown in Instructure's documentation to view inactive students.
To request that a student be re-enabled in a Canvas site, use the "Click to Request Service" button here and specify the student and the exact term, course and section. ITS will verify that the information is correct and then, upon finding that the student had been enrolled in the course previously and is no longer active, will process the re-enrollment.