Jon Shaulis builds a logon trigger to notify when sysadmins log into his systems:
I was helping someone set up some monitoring in their database and they were asking about being notified when someone with administrative privileges logs into SQL Server. While many of you know that I try to use the right tool for the right job, some may cringe when I say triggers can help out in this scenario.
Now, there are other methods here but the goal was to be notified when someone logs into your SQL Server with administrative privileges. Triggers will consistently fire when this event occurs and allows you to perform an action based on this event.
Just make sure you get the trigger right and don’t block everybody from logging in. That’s an awkward situation.