You might also be thinking of a Knox Box. A key securely stored in a firetruck can be used to open a small box on your property, which contains a master key fo

You might also be thinking of a Knox Box. A key securely stored in a firetruck can be used to open a small box on your property, which contains a master key for the facility. As scary as that sounds, it isn't /completely/ stupid. The system is designed to make using this backdoor is exceptionally noisy. Your Knox Box should have a tamper switch inside, so that any time it is opened, alarms go off. At the other end, when the key is removed from a firetruck for use, alarms go off.