The cop still has the statement of witnesses as proof of a crime being committed. If I witness a woman beating her child in the parking lot of a store, call the cops and give that statement and when they arrive the mother and child are now sitting quietly in their car, they have the right of law to detain that woman and question her child about a crime and/or examine him for injury. Just because the woman may have STOPPED beating her child by the time the cops come didn’t mean she wasn’t just doing it 10 minutes ago. The cops INVESTIGATE allegations of crime and then use their training to determine of said allegations fall under a chargeable crime. The cops will definitely ask that mother to provide ID as well to see if she has outstanding warrants or has been arrested for child abuse before. That’s their job in a nutshell.