A conundrum. (Or is it?)

As I returned to my flat yesterday, my near neighbour was pulling a sledge that contained his daughter.  When we went up the stairs he said, “Have you seen the spider’s web on the outside of the landing window?”
    “Wow,” I said, “that’s some web, but it won’t catch any flies.” I went out to look at it again this morning, but somewhat mysteriously it had disappeared.  I think I smell a rat.