My usual response is curiosity - but I was left in drop-jawed shock when a lady, who I assumed was a customer, just disappeared in a bookshop (I'm sure I was pale as well!)
Since I know someone will ask - yes, I have posted it on here -
original post (the second of the two accounts there). Text below (posted back in 2003):
More recently, maybe two years ago, I went to Methven's Booksellers in Woking, just browsing. I went upstairs and wanted to go down a narrow, dead-end aisle between a table piled with books and some bookshelves to look at the end section, but there was a lady down there looking at the books, so I stopped by the entrance to the aisle and looked at some other books while I waited for her to leave. I could see her out of the corner of my eye, flicking through a book. After waiting a while I got bored and decided to go down the aisle anyway...I looked up and no-one was there. The aisle was too narrow for her to have passed me without asking permission. I was, to say the least, very surprised. I looked through the whole upstairs section and she wasn't there, and I hadn't heard anyone go downstairs. She just looked like a normal lady, except (and I only realised this afterwards) that she had been wearing a heavy overcoat and a scarf, on a hot summer's day.