It's funny now but it wasn't then. I always keep the card inside my port and I noticed it wasn't there when the UK border officer was looking at the port. So then I looked on the van floor and under the seats but I didn't know for sure if it had even been in the port when I set off that morning, I'd checked I had the port but I hadn't looked inside, and when it didn't seem to anywhere in the van I assumed it had gone missing previously. I suppose the card must have pinged out without me seeing when I got my port out of my bag to give to the border officer, and it had landed lying against the inside of the metal seat frame, on its edge.. So it wasn't flat on the floor and visible, and I hadn't felt it with my hand when I reached around under the seat, the only way to see it was if you put your head upside down on the floor in the footwell and looked really carefully with a torch. Which at the port with luggage on the seat wasn't very possible.
The border guard was lovely, he was a sensible chap of a certain age, not one of the young aggressive breed. I tried to fob him off with my old EU cds which I still carry in my purse, but I could see by the way he was looking at it that he was smelling a rat so I itted I knew it was the wrong cds and explained I did have an Article 50 one but I couldn't find it. And I showed him a scan of it on my phone. And in the end he said Bon allez, I'll let you in without a stamp.
It took a while and the people behind me in the queue must have been hating me.