A day in the Cover Diary browse is showing in red but there are no periods requiring cover
This is caused by a staff member having two instances of absence, one absence was added and the cover was entered and then another absence has been added which overlaps the original absence.
See the example in the screenshot below
Mrs A Abell has an absence with the reason of Illness for the whole day and then a second absence for the reason Interview starting at 12 noon.
All her lessons have been covered and yet in the browse, the day is highlighted in red showing that 2 periods require cover.
To resolve the conflict select one of the absences and right click, choose Delete or Edit the times of the absences so that they no longer overlap.