Many years ago I worked for an alarm company and would troubleshoot and repair burglar alarms, whether from a false alarm or an actual break-in. One of the other tasks was to asist businesses that couldn't set the alarm when closing.
I was at one of those calls and there were never any schematics available. There was one interior door that was propped open with a metal milk crate. I couldn't find any alarm contacts on the door, and the customer told me that particular door was propped open every night anyway.
Another alarm tech, a young hot shot, stopped by to lend some assistance, as I wasn't getting anywhere. He walked in the front door, walked over to the propped open door, kicked the crate, sending it tumbling across the floor, and headed back toward the front door without saying a word and without breaking stride. The door's auto-closing mechanism slammed it shut and the green "ready" alarm light lit!
Moral to the story:
Don't accept any customer's "facts" as truth. Take the time to verify each possible part of the puzzle.
Steve