Whatt o do if I just found out that I have a warrant for my arrest for probation violation from 11 years ago?

You have several questions.  First, the phrase "non est inventus return of warrant" means that a warrant was once issued for your arrest and it was returned to the court unserved.  It is significant that this is on a warrant, because when this is written on an unreturned warrant, the clock essentially stops clicking on your probation and you get no credit towards your probation for any time after the entry of this warrant.

The second part of your question is about a statute of limitations for a probation violation warrant from 11 years ago.  In Georgia, there is not a statute of limitations for a probation revocation as long as the motion to revoke is filed before the probation expires.  In your case, if the warrant had not been returned "non est inventus return of warrant," and if your probation would have ended a year ago, then the state could not go forward with the MTR.  However, because of the "non est inventus" return, the State can still pursue the revocation.

You may, however, have a defense to extradition, depending on the rules of your state.  If your state does have "due diligence" rules, they may refuse to extradite you to Georgia.  Due diligence is not a "statute of limitations", but acts similarly in that it requires the state to do more than just "drive by your house and see if you are home."

The final part of your question is-- what to do??  You need to start by talking to an attorney in your current state that understands multi-state extradition laws-- because that's going to be your first issue.  Then, you need to see if he/she can help you obtain an attorney who is licensed in Georgia to help you.  Considering how old this case is, they may be able to negotiate for a withdrawal of the motion to revoke if you simply payoff your fees, fines, and court costs.