Ah, wandering shops.
Maybe 25 years ago I was walking the streets of Rome, as I had been doing for years on my days off. Maps were left behind years before - didn't need them. That day I was on the Tevere side of the Tiber (well, that would sound strange in Italian..), anyway, I suddenly came across a shop I had never seen before, or at least not noticed. In the window they displayed the most fantastic steam-punk coffee machine, with pipes going everywhere, but not that big either. I instantly wanted it. But it was a Sunday and the shop was closed so I decided to call it a day. It was just a short straight walk from the shop to a major intersection near the river, and then to a bridge and eventually to where I was going.
So I went back as soon as I could a few days later, walked to that intersection and up that road, about a hundred meters.. and the shop was not there. There was nothing on that corner. I thought I was losing my mind, or at least I had managed to mess up something.. after that I started out from every bridge on the Tiber, walked to the nearby major intersection, and up whatever street in the right direction. Nothing.
I continued my search for years after that whenever I came back to Rome, but I never found the shop. At that time I didn't really want to buy the coffee machine anymore, I just wanted to solve the mystery.
Obviously (if I think logically) there's a natural explanation, but it still comes back to me now and then, all these years later. Back when I read that Pratchett book in your link, which went into details about how a Wandering Shop behaves, I suffered major déjà vu!