In Diagnosis Murder: Misdiagnosis Murder (1996), Dr. Travis sees a man die of a heart attack in the parking garage. When he returns with aid, the body is gone and another man is faking tachycardia. Dr. Travis spends the rest of the episode trying to persuade people of what he saw.
We see Dr. Slone constantly ask his detective son to put out APB's (All Points Bulletins) or BOLOs (Be On the Look Out) to bring suspects and witnesses in. When found the police just walk up to people and place them in the squad car, taking them to the station for Steve and Mark to interview even though it's pretty clear they didn't want to come. If there is no probable cause for an arrest or an active arrest warrant, the police can't make you go anywhere against your will. Once in a while, a wealthy or educated person will assert this and ask a lawyer to be present but this is rare.