Why have alerts that were closed disappeared from the grid?

Alerts that were in the grid in the Warnings > Closed menu may have disappeared for two reasons:

After manually closing the alert, the system ran a check and validated that it had not been resolved. The alert was then reopened and returned to the Warnings > Open menu to be dealt with again.
Seven (7) days have elapsed since the alert was closed, and this is the maximum period that can be viewed in the Warnings > Closed menu. To view longer periods, you can generate a predefined report of closed alerts with the desired period.