Years ago, When I was working as a "Fast food" manager, the drill was to use a grease pencil to mark the expiration time of the baked potatoes on the front of the holding cabinet. In the ongoing effort to keep things "clean" employees were constantly wiping the times off. At the local Western Auto Store (I think they are now out of business) I found a little plastic gizmo with sticky back mounting tape on it that had four or five number wheels. The device was sold as a mileage reminder (just the purpose we are looking for here). I used them to indicate the expiration times. It would end up being the same as having one of the luggage locks BRGreen mentioned, but the cost of the plastic gizmos was under a dollar for two.