We often have the workflow situation where a tech will decommission a device without removing it from Orion first.
When the device is removed, it creates an alert for a down device (on the alerts screen). When the device is subsequently deleted however, the alert remains - the reset condition will never be generated to remove the alert (and the network object changes to a blank value on the alerts screen).
If a device is deleted from Orion, any associated alerts should also be removed.
Dave.