Re: I Believe I Was Unlawfully Detained
If this were one of my guys, I'd be all over them to study up on what constitutes reasonable suspicion and a lawful detention. This sort of action can cause the agency to lose a case or even get sued. Regardless of what bootstrap excuse the officer might come up with after the fact, if the incident is even half of what the OP describes it as, the detention was extremely weak at best, and entirely unlawful at worst. I suspect the latter.
Since the OP was told that the reason he had to stay was that he was being forced to stay and talk to the sergeant, and there appears to be no further investigation or even MENTION of some form of public disturbance as a crime, I don't think that this excuse would hold water and would be nothing more than a lame attempt by the officer to justify the unjustifiable.
The story as told indicates that the OP was detained solely to speak to the sergeant. Since not speaking to a police officer is hardly suspicion of criminal activity, there would appear to be no reasonable suspicion to justify any further detention.
A Nor Cal Police Sergeant
"Make mine a double mocha ...
... and a croissant!"
Walk humbly with your God
-- Courageous, by Casting Crowns