2006 CTS not starting "Service theft system" msg
 

So as title states my car a 2006 3.6 cts

Was running perfectly last used midnight or so yest.
Started and ran fine.
So I went to start today and all my accessories have power. So I know it's not battery. Nor the alternator because that was replaced in July.
I must admit for the 2 yrs I've owned this car I never had an alarm box and or a keyless entry remote. Only always used just a key. Is anyone familiar with this nonsense?
Thank you