He was likely a civilian doing a ride-a-long with law enforcement. It's not that unusual.
That being said, you should have your attorney find out who they were and have an investigator interview them for their observations regarding your sobriety.
Best of luck to you.
No answer here should be considered to form an attorney-client relationship. You should consult with a licensed attorney in your jurisdiction so that a full evaluation of the facts of your case can be conducted.
This type of thing is fairly common. You may want an investigator to interview the man if he didn't write out a statement, but in my experience civilians that do ride-a-longs with police are very pro law enforcement in their testimony. Your attorney can help you decide if they should be interviewed.
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First step: you need to get a copy of your discovery to see if this person was referenced in the police report or any other documents. This seems strange but not out of the question given so-called "ride-alongs". The bigger issue is whether/which rights you waived when it comes to voluntarily revealing information about yourself, your state or intoxication at the time of arrest or any other relevant factors.
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