The events list currently doesn't include a grades unhidden event. What we would like is for there to be one such as:
Area to monitor: Grades
Event: grades unhidden
We would like a notification to be sent when feedback is made available to students all at once.
The rationale is that it's usual for staff to hide grades for Moodle Assignments in the gradebook until marking is complete for the whole cohort. The grades can be hidden manually or hidden until a certain date. When they are unhidden this should be an event, available for monitoring via the Event Monitoring feature.
Similar tickets for other assessment tools include MDL-57233.