If you have setup a trigger campaign to send you an alert based on lead activity, you may have received a few blank alerts.
Anonymous leads qualify for trigger campaigns, this is by design, you want anonymous leads to be scored if they show desirable behavior, but receiving blank alerts probably does not make sense.
Use the Is Anonymous filter in your trigger campaign's smart list in order to stop anonymous leads from entering:
Excellent! You should no longer receive blank alerts from your anonymous leads.