As you said enroll a student is enough to make course changed for backup.
Try this query (mariadb,mysql) to see if at least they have actions logged in your database for that course.
FROM mdl_logstore_standard_log t1
WHERE courseid = 2
LIMIT 0, 100
I also use next query to make an HTML report with all the courses backuped today :
SELECT DISTINCT t1.courseid
,DATE_FORMAT(FROM_UNIXTIME(laststarttime), '%Y-%m-%d %H:%i') AS start_time
,DATE_FORMAT(FROM_UNIXTIME(lastendtime), '%Y-%m-%d %H:%i') AS end_time
,SEC_TO_TIME(lastendtime - laststarttime) diff_time
FROM mdl_backup_courses t1
LEFT JOIN mdl_course t2 ON t2.id = t1.courseid
WHERE DATE_FORMAT(FROM_UNIXTIME(laststarttime), '%Y%m%d') = CURDATE()+0
ORDER BY laststarttime DESC
You can also with the first query make one that give you the list of courses that should be backuped...
Hope it's help.