A self checkout weighs the bagging area against a database of expected weights, so the security exception it raises has a number on it. Till 4 raises one every night at about ten past two, three hundred and forty grams, with the store shut and the tills locked.
I am a duty manager at a large supermarket and I have done the overnight shift for seven years.
How the machine decides something is wrong.
The bagging area is a load cell. When you scan an item the system looks up its expected weight and waits for that weight to arrive. If what arrives is outside the tolerance you get the message everybody in this country can recite, and the exception goes into a log with the till number, the time, the expected weight and the actual.
That log is not for us. It goes to loss prevention, because the pattern of exceptions on a till is how you catch somebody scanning a cheap item and bagging an expensive one.
So every exception has a number in grams on it, and a trained person reads those numbers.
Till 4 raises a security exception at between ten past and twenty past two, most nights.
The store closes at midnight and opens at seven. The tills are powered but locked, which means the screen shows the closed message and the scanner is off and the system will not take a scan.
The exception is logged as an unexpected weight in the bagging area, with no item scanned.
Three hundred and forty grams.
Not about. Three hundred and thirty eight to three hundred and forty four, which is the repeatability of that load cell.
I have four hundred and eleven of them in a spreadsheet because the exception log exports and nobody else has ever wanted it.
What has been done about it, and head office has paid for most of it because a till that raises phantom exceptions corrupts the loss prevention data for the whole store.
The load cell. Recalibrated with test weights twice, and then replaced, and then the whole bagging unit replaced in 2022.
The till. Till 4 is the third machine to stand in that position. The one that was there in 2019 is in a store in Leeds and it has never done it there.
The floor. A load cell will read a vibration, and the classic cause of a phantom weight is something shaking the frame: a cleaner's machine, a trolley, a fridge compressor cycling through the floor slab.
We tested that properly. An engineer put an accelerometer on the frame of till 4 for a week in 2021. There is nothing at that time of night. The floor is quieter then than at any other hour, which you would expect in a shut shop.
The software. Version updated three times since it started, and on the 2023 update the exception threshold was changed, and the exception still comes up because three hundred and forty grams is well over any threshold anybody would set.
What three hundred and forty grams is, because everyone asks and the answer is nothing useful.
It is a bag of sugar short of a bag of sugar. It is a large tin, a loaf, two pints of milk at a push.
I have put things on that plate and watched the number. A 500 g pack of mince reads 498. A four pint milk reads 2310. Nothing in this store weighs 340 on its own except, as a colleague pointed out, a standard hardback book and a bag of carrots.
The log does not say what it is. It says what it weighs, because that is all a set of scales knows.
The thing I did in 2022, which I have not put on any report.
The exception has a duration. The system records when the weight arrived and when it went, because it clears the exception when the plate returns to zero.
The weight sits there for between four and nine minutes and then it is gone.
So I went and stood there. Twice, in March 2022, at two in the morning, with the lights on the half setting they go to overnight and nobody else on that floor because the night fill team work the far aisles.
Both times I was at the till before the log says the exception started and I watched the plate.
There is nothing on it. I mean that flatly: the stainless steel plate is empty and I had my hand on the frame and my eyes on it and the screen behind it showed the closed message.
The second time I had my phone on a tripod filming it, because I am not an idiot and I knew what I was going to be asked.
The video is four minutes of an empty bagging area. At two fourteen the screen changes to the exception message. Nothing comes into the frame and nothing leaves it.
I have that file. I have shown it to one person, a regional loss prevention manager, who watched it twice and then asked me, carefully, whether anybody else had access to the log export.
What we do now.
Till 4 is flagged in the system as excluded from loss prevention reporting, with a note that it has a recurring equipment fault, and the exceptions are filtered out of the store's figures.
That filter is the only reason this is not a problem. Our store's exception rate is a number somebody is measured on, and without the filter we look like a store with a thief on till 4 every night for seven years.
What I do.
The round goes up the front of the store and back along the checkouts at about half one, and then again at three.
I have moved the second one to quarter past three.
And I have stopped taking the short way back to the office, which goes between tills 4 and 5, because in November 2022 I came through there at about two twelve with a clipboard and the plate on till 4 was empty and the screen was still showing the closed message, and as I passed it the scales under my right hand took something, because the frame gave the way a set of scales gives, about a centimetre, the way it does when you put a bag of sugar on it.