so what if at first you announce that a camper is missing, and get all of the staff's reactions and feelings. Then announce that it is actually a drill, but tell them to react like this is a real deal. maybe give them a time limit and if they do not find the camper in the set time, or something goes wrong, that they will have to do it again. End the drill on a good note with a coupon for more time-off one day, pizza, etc.