Citizen entrapment

As I understand it, people who break the law cannot be prosecuted if the police set up the situation in which they break the law. Is the same true if the general public rather than the police set up the situation? What if, a group of citizens leave a briefcase in the back on an unlocked car, but hide video cameras and put a tracker in the briefcase. Once someone steals the briefcase, it is reported to the police and the tapes and tracker handed over. Is this allowed? Is it acceptable?