You don't need an operator set to trigger an alert.

I have a test alert set as follows:-

EXEC msdb.dbo.sp_add_alert @name=N'Test Alerts',

@message_id=0,

@severity=16,

@enabled=1,

@delay_between_responses=0,

@include_event_description_in=0,

@event_description_keyword=N'Test Alert Error',

@category_name=N'[Uncategorized]',

@job_id=N'00000000-0000-0000-0000-000000000000'

GO

I then test it using the following:-

RAISERROR('Test Alert Error', 16,1) WITH LOG

The error is raised and written to the SQL Server Log, but the alert is not fired (checking the alert history number of occurrences remains 0).

I have then created the exact same alert on another server, and this does fire and increments the alert history.