Better yet, why not fit some of those blinking lights on each end? If the hinge pin is not within the confines of the frame, the lights will come on and blink madly until returned to the proper union with the frame and barrel.
Maybe it could have a tiny internal "buzzer" that comes on when removed from the frame so that we could listen for it in the event that we are ever changing barrels in pitch black dark and drop it. If we go blind from all the fun we're having from shooting Contenders, we could just listen for the "bzzz-bzzz-bzzz" until we find our misplaced hinge pin!