I received a felony DUI last year and have just finished fighting my case. I was on formal probation when I was detained in another county for public intoxication under penal code 849.5, but was not given a breath or blood test. Then I was released 5hours later. Can my probation find out even if this occurred out of county and the DA does not file charges against me in the county I received the public intoxication in?

Yes, while not available for public view, for police and law enforcement, even all arrests show up on the NCIC report.
Plus remember the biggest violation of all will be not reporting it, So when they find out, the major issue will not be being held in the drunk tank, it will be not reporting it.