Add Mailbox Permission NT AUTHORITY\SYSTEM

This alert occurs once or twice a year to all Microsoft tenants and is a normal reoccurring event. 

The event is where Microsoft is refreshing or regranting themselves full permission (Full Access) to the Discovery Search Mailbox.

Here is an example of the event description:

NT AUTHORITY\SYSTEM (Microsoft.Exchange.Servicehost) made the change for YOURPRODNAME.prod.outlook.com/Microsoft Exchange Hosted Organizations/colorriteco.onmicrosoft.com/DiscoverySearchMailbox{D919BA05-46A6-415f-80AD-7E09334BB852} as FullAccess