This alert will inform you as databases are added or dropped from your servers.  With backup requirements, and space requirements being what they are, it's important to know if any DBs have been added so you can make sure you've got plenty of disk space for all operations including maintenance and growth.  As well, it's equally important to know when DBs have been dropped.
These alerts aren't actionable as they're meant to merely give you the information.  So there are no deferments, or thresholds to configure.  The alert will only be sent once for any set of collected DBs.

How it works:

The table dbo.DatabaseList contains the current list of all the DBs on all your servers.  The first time this alert runs there's nothing in the table.  Therefore, the first time it runs every DB in your shop will be added and the alert will consider them to be new.  So you'll get an alert with all your DBs marked as new.  From there on out, the DBs that get collected in Collector.DBProperties will be compared with this list and the ones that aren't already in dbo.DatabaseList get added to it and marked as new, and the ones that are in dbo.DatabaseList but aren't in Collector.DBProperties get removed from dbo.DatabaseList and marked as retired.

The reason the DatabaseList table is necessary is because you could have several DBProperties collections before this alert fires.  You could be collecting DBProperties every 30mins and only running this alert once a day or even once a week.  So there could be many changes lost because you don't know which collections to compare.  So we have a "gold standard" master list of DBs.

There's nothing in this system that certifies or verifies the DBs in the DatabaseList table.  So if you wanted to add or delete any rows yourself for some specific reason, the system will continue to operate as usual and won't know the difference.  One reason for altering the DatabaseList data yourself would be for testing the alert.

Limiting Results:

There may come a time when you want to not alert on some DBs.  Maybe you've got a server that has quite a bit of fluidity to it and you don't need to know about every single DB that comes and goes.  You can control which DBs you don't get alerts for by altering the dbo.SearchRegex table.  Add a row for this alert by setting Module = 'NewRetiredDBs'.  For more information on the dbo.SearchRegex table, see this article.