Mystery At M&S

On Friday afternoon I dropped in to Marks and Spencer's to do some shopping on my way home from the station.  At the checkout something happened that I have never come across before.  The operator scanned in a cheese and onion flan and the checkout displayed the message "Not allowed to sell this item".  He scanned it again and then put the flan to one side.  I asked him what was up but all he said was that he wasn't allowed to sell me the flan.  From the attitude of the operator it seemed it wasn't the first time it had happened today. I looked around, several other checkouts had items set to one side. I checked the sell-by date on the flan: it was the 18th of July, so that wasn't the problem. Slightly mystified I packed my bag, paid for the rest of the items and left the shop.

On the walk home it occurred to me that the most likely explanation was that M&S had just learnt that one of their suppliers had a problem with their production process (contamination or temperature control, or something) but that M&S had not yet had time to take all the affected products off the shelves.  However, one thing did not quite fit with this: I had distinctly heard one of the checkout operators asking an assistant to put a handful of what I presumed were set-aside items back on the shelves.  Anyhow, I put it all to the back of my mind and when I got home we had an omelette for tea instead of cheese and onion flan.

Then, on Saturday I heard  on the radio that several branches of Tesco's had been closed for the afternoon.  It wasn't said explicitly at the time, but to me it sounded like this was in response to attempted extortion.  I wonder if the M&S mystery was a result of the same extortion.  The low key of the M&S response would then have been down to their computer system being able to flag suspect items at the checkout.  Tesco's computer system might not have been able to do that, and so they would have had to shut their stores while they searched their shelves.  

Still, this doesn't tie in with the request to put stuff back on the shelves that I overheard.  But maybe that was just a red-herring.  Maybe a customer was so miffed at unable to buy one item that they just abandoned all of their shopping at the checkout.  The operator would then have asked the assistant to put the safe items back on the shelves.