On Sep 19, I saw a tip for using a contact sensor to indicate the locked or unlocked state of a deadbolt by nerdland. Below in pictures is my approach. It does not involve much door surgery, only a small 3/16"ø hole drilled halfway through the wall and a modified contact sensor sending unit. However, the modified sensor is attached to the outside of the wall and is visible. I have used a version of the modified sensor to monitor my steel mailbox (see thinking outside of the steel maillbox part 2) activity and I am sure there will be other adaptations. Yes, mailbox is misspelled - but that is the title.
Modifying the sensor is fairly easy but is not covered in this presentation. I will provide that later.
The case itself is attached to the wall so the door can be opened to change the battery without putting stress on the leads.
The sensor is destined for another client and was mocked-up tested. The drawing depicts the geometry of his door.