The course statistics reports shows "guest" activity for actions that were taken by the system, e.g., (automatically) updating an aggregated grade as the consequence of a (manual) grade change. This affects "Reports > Statistics", but might be visible in other reports, too.

We realised this by looking at statistics of a course that had high interaction by guests, even though guests were (definitely, we checked) not allowed to enter the course. Statistics looked like this (guest = orange):

After some investigation, we found some artificial grading events to be the cause. Those events were attributed to user id -1 (meaning: System user - USER_OTHER in \core\event\base). I suspect code such as that in https://github.com/moodle/moodle/blob/a63cd3e2ca8f864a832c5e050677acbf1bda1701/lib/statslib.php#L493 might be the reason, because a user "-1" is definitely never enrolled in any course - but I don't know if that is the actual root of this particular issue.

Expected behaviour:

• Attribute actions only to guests if they were performed by actual guests
• Do not show system user actions in this graph

Steps to reproduce:

1. Create a course with an activity
2. Enrol a user as a student into the course
3. Make sure guest access is disabled for course
4. As an admin/teacher, go to the grade report and override the grade for the user for the particular activity
5. Update your system time to day+1 - Needed to do this so it would be picked up in the stats report mentioned
6. Run 'stats_cron_task'
7. Go to the course's "Reports > Statistics"
8. Notice 'Guest' activity in the graph and table below.