If an employee has received an email regarding another employee's sickness but they are not their manager or approver, this means the notification may have been set up incorrectly.
To amend it:
Go to Setup and click Settings.
Click Notifications.
Look for a notification called 'Sickness to Close' and click it.
Click Edit in the Emails area.
Make sure the recipient has been set to Reports To. This ensures that only an employee's line manager will receive this email.
If you do not want anyone to receive this, click Delete.