Rolling stacks move on a rail when somebody presses a button, and the controller logs every move and every motor's run time. Bay 14 opens between two and three most nights, in a locked basement, and in the morning there is a gap in the shelf at eye level where a book has been taken and put back.
I am the stack supervisor for a research library, and the basement stacks are mine.
How the shelving works.
Twelve bays of stacks on rails, each bay a run of shelving ten metres long. There is one aisle at a time. You press the button on the bay you want and the stacks roll to open it, with safety sensors along the floor that stop the motors if anything is in the aisle.
The controller logs each open: bay, time, duration. The motors have hour counters for the service contract.
The basement is locked at seven. It is a strongroom with one door and no windows, and the door is on the alarm.
Bay 14 opens between two and three most nights.
The log shows the request from bay 14's own button, the stacks either side rolling, the aisle held open for between four and eleven minutes, and the aisle closing.
The alarm log shows the door shut all night. The motion sensor in the basement, which is separate from the stacks' floor sensors, records nothing.
The stacks' floor sensors are the part that made this mine rather than the alarm company's.
They are pressure strips and beam breaks along the aisle floor, and they exist to stop the stacks closing on a person. The controller logs when they trip.
On the nights bay 14 opens, they trip. In the aisle, about six metres in, for the length of time the aisle is open, the way they trip when somebody is standing there. And when the aisle closes, they do not trip, which means whatever was standing in the aisle for eleven minutes was not there when the stacks came together.
What is in bay 14.
Local topography and the unbound pamphlet collection, which is the part of the collection nobody asks for. Six metres in on the left is a run of bound volumes of a defunct county journal.
Most mornings after a night open there is a gap on that shelf. A volume pulled forward and pushed back not quite home, the way a book sits when somebody has taken it down and replaced it. Always the same shelf, and the volume varies along it, and I have started recording which.
They are being read in order. One volume a night, along the shelf, and in 2023 the sequence reached the end of the run and started again from the first.
What has been checked.
The controller replaced. The button on bay 14 replaced and then disconnected: the bay opened that night with the log showing a request from a button that was not wired to anything.
The rails and the motors, in case a stack were creeping on its own. A creeping stack does not open an aisle of exactly one bay's width and hold it.
The alarm and the door. The door has a magnetic contact and a camera on the corridor outside. Neither records anything. The basement's own motion sensor, which sees the whole floor from the ceiling, records nothing while the floor strips six metres into the aisle are reporting a person.
What is done about it.
It is a logged event on the shelving controller with a fault code the manufacturer invented for us, and the service company checks the motors every quarter and finds them fine.
The journal is not requested by anybody living. I have checked the request slips back ten years. So the reading disturbs nothing and the volumes are shelved by me in the morning.
What I do.
I supervise the stacks and I do the morning check.
The one change is that I do not use bay 14's aisle myself after five in the afternoon, and I have told the two assistants who have basement access the same, with a reason about the floor sensors being unreliable, which is not true.
The reason is a Wednesday in 2023 at ten to six when I was in bay 14's aisle re-shelving a volume, six metres in, on my own, and the aisle began to close.
The floor strips were under my feet and they should have stopped it and they did not, because the controller's log shows they were not tripped, which means the controller did not see me standing there. It saw the aisle as empty. It closed to about a foot either side of me before I got to the end of it, and the log for that minute shows one thing on the floor sensors, six metres in, exactly where I had been, registering after I had left.