You can buy alarms that are like the alarms for home alarms that go off when the door is opened. Like one used for windows. You can get them at the hardware store. They can be turned on and off so if they wanted them off at a point people were going in and out, they wouldn't be getting the alarm all the time and then turn it back on when they want it secure. I was an escape artist as a kid. My aunt sleep walked so they had to put extra locks on the door as one night she slept walked all the way to a friend's house. There is a history of sleep walking in the family.