When the PTA lady sees Helen on the street outside the market, a yellow Chevy Nova II is seen parked, on the street, in the background. A minute later, it is seen driving past them as they chat. A few seconds later, it is still parked on the street behind them.
When Helen tells Aunt Bee and Opie she's going to the school board meeting she turns right as she walks out the front door. There's no way down from the porch on the right side, just a dead end. This confirms that they are in fact filming at the Desilu studios and not on location.
Andy goes to some lengths to get information on Helen's arrest and charges from the archives of the newspaper that reported the story. However, as a law enforcement officer, he could have obtained the full details more directly through official channels.